What size backpack?

Best to get all the rest of your gear first, then box it up, haul it down to the store, and find the smallest pack that'll fit it all (including N days' food).

Several gear decisions have substantial impact on the size pack you need. For example, if you use a closed-cell foam pad, it's likely to get strapped to the outside of your pack, not in it. But an inflatable will probably go inside. A down sleeping bag will compress smaller than a synthetic. Will you use a bladder for water or carry bottles? The former will require a sleeve and room inside the pack, the latter good side pockets. Will you use a water filter, chlorine tablets, or something in between? Very different space requirements. Some tents take no more room than a Nalgene bottle, others fill half a good-sized pack.

And on and on. Buy your pack last if at all possible.

I used to have a Jam2 which is basically the old version of the Jam 50. I'd go for the Jam 50 because it is much more flexible. You should be able to carry a weeks worth of food in it. On shorter hikes the compression system does a very good job of squeezing the pack down so there isn't a lot of excess volume. The Jam 35 on the other hand is very small and has a short extension coller which is annoying.

By the way a therma rest is not an idea pack support for a frameless pack in my opinion. I think it would be worth putting a a folded over peice of foam pad in the pack. It could be a whole pad or you could use half a pad and combine with the theramrest.