Tarp size when used with a bivy

Maybe, the constant answer. The lower and flatter the tarp is pitched the better the coverage, but the less livable it is. A poncho will keep you warm and dry with good site selection, but you will not always be able to sit up.

Was that vague enough?

For me, length is more of an issue then width – I'm a tall skinny guy…My tarp/poncho measures 55" x 110". I can pitch it right to the ground (guessing 30" height or so) with my poles fully collapsed and still "slither" inside when the weather gets really nasty. I'd get a smaller one, but really, with the crazy light weights of some fabric nowadays, it's hardly worth it. A 5×8 is wider then my setup, so IMO you should be good to go – however, the length would stop ME from using it (how tall are you?)…gotta have room to put your boots too!

Golite Poncho Tarp measures: 8 ft 8 in x 4 ft 10 in
is big enough for my 5 ft 11

I've used it with Bozeman Mountain Works VAPR Bivy Sack

Excellent results for me so far.

I'm looking forward to trying Mountain Laurel Designs Silnylon Pro Poncho – its shape tapers front to rear (67’ wide front / 55” wide rear / 9’ (108”) Long). Should be getting it soon per the shipping estimate when I ordered it.

I cut my 8×10 foot tarp down to 6×10 feet. It saved 25% of the original weight, and provides plenty of coverage for me and my gear in any weather. I also have a Golite poncho tarp, and when pitched it's just not quite long enough unless I "raise" the edges about 6 inches off the ground. (I'm 6-2.)

It's the size I use with my ID Unishelter & it's fine. The Unishelter is enough for most conditions on its own, the tarp is mainly for dry entry/exit & cooking. With the setup shown below, I stayed dry in heavy rain & 50mph winds overnight in Patagonia.