What is a good rule of thumb for tent floor length relative to height? I'm tall and looking for a backpacking tent. How many inches more than my height should I look for in the floor?
There's really no simple answer to this. Here are some variables that people sometimes overlook...
Are the walls sloping or vertical?
The first factor to consider is the slope of the inner at your head and feet. In some designs the walls are vertical. But in others the the space can narrow quite sharply with height - add a thick sleeping mattress and suddenly you have much less room than you would expect from the floor plan.
On some tents with sloping walls the inner can sag quite badly, especially in damp conditions, so you might need more space to keep if off your face. Some people hate having the inner close to their face, while others don't seem to mind.
The thickness and type of your sleeping bag
A big winter bag can have considerable loft in the foot-box and the hood, so allow for this in addition to your height.
If you're camping in high condensation conditions and you have a down bag, you might want a good margin of safety to prevent the bag from soaking up condensation from the tent outer.
Ideally, get advice or try it out
If you're buying from a specialist cottage maker, have a chat with them and they will advise you - they may even have larger models for taller customers.
If you're buying something mass market try to find a store that will put up a demo for you and try it with the gear you'll be using, bearing in mind that things will often sag more in the outdoors.
It's also worth going on the backpacking forums and simply asking people how they are getting on with the tent you're considering.
Better safe than sorry...
But in general, go for something with a good margin of safety - a tent purchase is an expensive mistake if it doesn't work for you. Don't fret over the odd extra ounce if you have to get something bigger - a tent you don't fit into is a false economy, I think.
With the assumption that you are backpacking, I would recommend height plus six inches, with the caveat to make sure there is some room to the sides. If you are car camping, then just get a tent palace and enjoy it ;)