Storage space in bivy? Bivy size question…

Last summer I foolishly set out on a two week trip with a bivy that I hadn't used in 20 years. I've gained about 30 pounds in that 20 years.

It was way too tight. Once I squirmed in (no zipper) it was okay in colder weather, but not in warm weather. There wasn't enough room to get out from under my sleeping bag on 50 degree nights.

I suggest pinning or sewing a mockup out of old sheets and trying it out for a night in the house. Get undressed/dressed, imagine a blizzard or monsoon, etc.

I plan on making a mockup this weekend. My tarptent-for-2's floor is only 86" long and my pack generally goes at the foot of the tent. So i feel the 98" length is excessive, while the shoulder girth seems very conservative. The pack cover can be used as a water proof floor for anything that was in the pack and isn't at night, but under the tarp. The basic plan for use of the bivy is under a 5×9 or 8×10 tarp and not by itself. Momentum top fabric and a silnylon floor, so it won't be a standalone shelter in washington state unless it's a dry summer night. Sunday i plan on making a prototype and tweaking it a bit, but i was just wondering about what dimensions for length and such, were other peoples bivys. The last bivy i used was a gortex GI Issue during basic training in the Marine Corp.