new to backpacking. how big of a pack do I need?

you need a pack big enough to hold all your gear, Plus 7 days food and as much water as you need to carry too.

Without knowing what gear you have, and are planning to bring, no one can really tell you.

Some UL backpackers, could get by with a 30L pack. Some need a 70+L .

More info required.

I have a
Suisse Sport Adult Adventurer Mummy Ultra-Compactable Sleeping Bag
Therm-A-Rest Trail Lite
I don't have a tent yet so if you have any recommendations for a light 1 person tent please let me know.
i have a 3 L camelbak
i'm planning on the rest being food and maybe i extra set of clothes and some small stuff like a knife, toothbrush, ect.
I am in the process of going raw vegan so I'm not going to bother getting a stove so the most cooking stuff I'll need is maybe a bowl and spoon along with my knife.
Once again, I'm new to this so if I'm obviously missing something then please let me know.

Buy your pack last. Take some time to look at the gear lists posted here and compare their gear with yours and pay attention to the pack they are using for the gear on the list. Volume and weight are both issues.

Unless you are starting out with a brand new set of the lightest and most compact gear, I would be looking in the 65 liter range for a week. It doesn't take any more gear to go for a week than a weekend, but the food takes up space. The bulky bits like insulated clothing and sleeping bag/quilt will suck up a lot of room unless it is uber-light stuff and very compressible. And then there are bear cans…

REI still has the Flash 65 on sale at $105. If it doesn't work out, you can return it. If you really get into hiking, you may find yourself with several packs depending on the duration of the trip and the season, and having a 65 liter around might be handy. It has enough suspension to be kind to a newbie learning how to pack gear and handle a little more weight. IMHO, if you think you need a pack bigger than 65 liters, it is time to do some serious work on your gear list.

Money aside, you could split the difference with an Osprey Exos 58. I *like* spending other people's money ;)

Indeed, get all the other stuff first then get the backpack that fits it. There is also no way of knowing how big a pack is untill you fill it, as listed volumes are very unreliable.

Get the new edition of Chris Townsends backpacking book or Andrew Skurka's to get a good handle on selecting and using gear.

Have fun!

Vegan/raw for a week might be a challenge. Dehydrated, no problem, but raw=water=heavy+bulk. Get a book on foraging ;)

Not the Hornet 46. Not with your other gear, and not for a beginning backpacker. It's a more specialized piece of kit.

Something in the 60-65 liter range, with a real internal frame, less than 3 pounds, should suit you for a long time. That REI Flash 65 is a great start.

Take all of your stuff (including everything but food)–for whatever season you intend on going out in–and a large cardboard box. Put everything into the box and level it as much as possible. Measure the box's width/length and the height of your gear, do the math to find the volume, and voila!, you've got a rough estimate of your gear's volume.

I usually figure about an extra liter per day of food/fuel using freezer bag cooking methods–this is probably high, but better to have a little extra space than not–which should give you a rough estimate of how much space you'll need for up to a week's time. Most places on the east coast, you won't be out longer than a week for resupply, though that number can and probably will go up if you go out into a real wilderness area for a long period.

Anyway, that's the easiest method I know of to estimate pack size.

I'll second John's suggestion. Pack as if you were going on the trip. Measure your volume, and you have your answer.
Obviously you will need to buy that tent first, and figure out what misc. items you will be taking.

Switch to raw vegan, after your trip…

Regarding your tent, how much are you willing to spend? Will this be one of very few backpacking trips in your life, or do plan to start backpacking on a regular basis?

You can spend $420 on a 12oz. tent, or $45 on 3.5lb. tent. And everything in between!

I forgot about that, the easy way! Doh!

You don't have room for "extra" clothes, no matter how big your pack is. What you DO have room for is layers with complimentary functions. An efficiently packed backpacker is like on of those nested Russian doll figurines; every time you pull one layer off, another one pops into view. Everyone's kit is slightly different (partly out of personal preference, partly because we're all hiking in different climates), but in general this is what the well-dressed backpacker is carrying:

Skin layer: For men – synthetic underwear, socks. For women – synthetic panties, sports bra, socks. Take 2 sets of undies; that way you can wash one set out and hang it on your pack to dry while you hike and still have another set to wear. (Some people forgo this layer and hike commando. As I said, personal preference!) Take 3 sets of socks: one worn only at night to sleep in, and two for hiking. Rinse out the used pair nightly to get the sweat out, and hang it on your pack to dry. Clean feet are happy feet!

Base layer: Long underwear top and bottom, made of synthetic material, silk, or wool. Used mostly as PJs, but if it gets REALLY cold during the day, you can also hike in these.

Main layer: Synthetic pants (long or short – depends on season and terrain you'll be hiking through), synthetic, silk, or wool shirt (long or short sleeve – again, depends on season and terrain).

Insulating layer: Insulating top and insulating pants. The top may be a jacket or a sleveless vest, depending on conditions, and may be wool, a synthetic puffy, fleece, or a down puffy. Ditto for the pants (which may be optional, depending on how cold you expect it to get during your hike). The insulating layer should be sized to fit over your main layer.

Rain/wind layer: Rain jacket or poncho + wind jacket, rain pants. These should be sized to fit over your insulating layer.

As I said, there's some flexibility depending on climate and personal preferences, but those are the basics. Think layers and NO cotton, and you won't go wrong.

So what do you do when it's Day 4 of your hike and your main layer is stinking like crazy? Well, you may be able to do some trail laundry. Wear your insulating layer or your rain gear, your skivvies, or (if it's really warm) just your birthday suit while you wash out your other clothes and while your main layer is drying out. (Hey, you're in the backcountry! Who's going to see you?)

I apologize if I repeated information you already knew, but too many new backpackers aren't accustomed to thinking like this, and accordingly they end up packing way too many redundant clothes. That will make any pack heavy fast! And when they've gained a bit of experience, they find themselves stuck with an overly-large (and overly-heavy) pack, because they bought a great big one in order to accommodate all those "extra" clothes they're no longer carrying. Efficient use of layers is one of the major keys to keeping your pack weight low and your back happy.

I might be a minority here, but since I don't own lots of packs I bought a fairly large one many years ago, a Golite Trek weight about 2 lbs, to handle the largest variety of conditions. It scrunches down if it's not fully loaded. The weight penalty seems minor compared to the cost of buying additional packs. It can handle a bear canister and a weeks worth of food, but is also ok (not perfect) for a short trip.

When it wears out I'll probably get a Golite Pinnacle or similar pack in the 1 to 1 1/2 lbs range. If I win the lottery then I'll get a whole series of packs, with weights starting at 8 oz, a different one for every conceivable type of trip.

To be honest, when it's just me and my wife I can use one of my kid's packs, a Golite Speed (about 1 1/2 lbs), that can handle a bear canister.

Skylar, here are two articles I wrote for Backpacking Light that might be helpful:

What a Beginner Needs for Backpacking Part 1

and

What a Beginner Needs for Backpacking Part 2

I kind of went through this when I bought a new pack a few months ago.

I ended up getting the Osprey Exos 46. It's JUST big enough for all my gear, and a little tight in the winter, but it works!

It is not as light as some on this website recommend, but its an awesome pack.

I know 4+ people, including myself with the Aether. 60L is the smaller of 3 capacities (60, 70, 85).

I have the Aether 60 and another 38L pack in my arsenal. I almost love this pack more than my wife :)

For a week, and being new to backpacking, you'll have a hard time with less than 60ish liters.

It is a little heavy but the extra weight is built in Comfort!
So I agree.. Awesome pack.