Annie - Not sure. I'd email Zpacks and ask them, they would know best. Your height and weight seem to me like you'd be good in a medium length and regular width, but also keep in mind that it's good to have some room for layers and maybe a few pieces of gear or whatever.
Michael - You know what. Your post actually motivated me to get up and test out this caterpillar theory. Got my bag out, got inside, and tried stretching it as much as I reasonably could. And it's somewhat true in my case, actually, though I never noticed it before. I am 6ft tall, so the long length is *just right* for me. When I stretch the bag out while inside, the last baffle rests right below my eyes. I never do this when I am out in the field, obviously. So I never really noticed or thought about the baffles being stretched, albeit slightly, which I suppose translates into slightly less efficient warmth. Then again, because I use this bag generally from late spring to early autumn when it's warmer, a tiny bit of baffle stretching is probably not enough to notice in these conditions.
Anyhow, I have an inch or two of extra space in my bag, so that when I normally sleep my feet are touching but not tight on the footbox, and the drawstring is snug and comfy around my neck.
But now I am split about what size to get for my 20f/-7c bag. I wonder if they could do an in between size? Because when it is colder out it's all the more important to have the best loft possible for most efficient warmth, of course. So squishing into a fatter caterpillar might be the way to go for my next bag with a x-long length (or maybe a custom "long plus" length?).
Anyhow, still quite happy with my sizing for my 40f/5c bag. It fits me spot on, but for a warmer/thicker bag, having more length would be much more important. Good to know. Thanks for this caterpillar tip!