The optimal length of a sleeping bag

Hello
I kept reading about sleeping bags on different sites and came across two ideas:
1. A sleeping bag should be 20-30 cm longer than the user's height.
2. It is not good for the bag to be longer than the user's height because it creates air gaps that are harder to heat, so you may be cold. It must be as tight as possible on the body.
From your experience, which idea is correct?
What would be a good ratio between the person’s height and the length of the bag?
What would be the advantages and disadvantages of a bag a few tens of centimeters longer than the height of the user compared to one of only a few centimeters?

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.

Yup, I agree with OM. Your "sleeping height" is taller than your "standing height" because your toes extend when you sleep. A quality bag manufacturer will take this into account and size there bags according to your standing height. If you plan on making the bag yourself then your going to need to make it longer than your "standing height".

You don't want dead space at the end of you bag because then your toes will have to heat up that air. Get the bag to fit your needs.

I also agree with OM. Unless you make the bag yourself, or can find someone to custom build it, you're going to be choosing between a short, medium, or long bag. Each will give a height range; just choose the one that is taller than you are. For example, I'm 5'10", so my choices are usually short ("fits to 5 feet 8 inches"), regular ("fits to 6 feet"), and long ("fits to 6 feet 6 inches.")

I choose the regular, even though it's about 2 inches too long. If I chose the shorter bag, my feet would press against the bottom of the bag, compressing the down there so it's not insulating my feet. In the regular, I've got plenty of length, and I've never noticed the "extra space to heat up" to be a significant problem (or even noticeable.) Some people I've camped with do get cold feet; they simply stuff a down vest, wool shirt, etc., to fill up the excess space, and the problem is solved. I used to carry down booties when it got really cold, which worked very well.

I don't know what conditions you'll camp in, but you might consider a quilt - essentially a sleeping bag without the hood or the underside. Without the hood, you don't have any real issues with the foot; you just put it loosely around your feet. Length is only an issue to the extent that you want your shoulders covered; if it's too long, just tuck the excess around your shoulders. You can extend the temperature range of the bag by also sleeping in a hooded down jacket and down pants (and down booties, if your feet are cold.)

Thank you for answers.
Maybe in US, but here in Europe we have all sorts of sizes. Starting from 165-170cm you could find it at 180/190/200/210cm and so on.
I was thinking of a summer bag with a 10°C comfort temperature, and I don't think that I will use it at altitudes above 700 meters.

Because cold is of minimal concern at 10°C or warmer, I would prefer a sleeping bag large enough to ensure I did not feel unduly constricted. You can easily wear something in the bag to compensate for any chill you might feel at 10°C.