As I found out a few years ago when I had bad plantar fascitis, when you are fully relaxed (as in sleep), your feet go into the pointy toe position (I didnt recover until I started using night splints). So if your sleeping bag is the same length as your height, your toes wil be pushing the foot of the bag. Besides, if you are wearing a fleecy hat and thick sleeping socks (which you will on cold nights), you need an extra inch or two to accomodate them.

This sets up a situation where if you are just an inch too tall (for example) a medium bag and way too short for a long bag, you have to decide. I'd get the long bag in this case., uness you always sleep curled up.

I bought long bags for my grandkids (figuring they'd grow into them, which they did) and, while they were still small, tied a string around the foot to eliminate the extra air space. Once they hit their teens, the extra space vanished!

In freezing weather, you'll need to share the bag with your water filter. your fuel canister, and possibly a hot water bottle and battery-operated items, so be sure to allow enough room for these items.