In British usage at least, you would generally use foot in expressions like:
Five foot ten
Meaning five feet and ten inches, often written 5'10".
However, if you actually use the word inches, you can use feet:
Five feet ten inches
But you don't have to:
Five foot ten inches
That's just an odd behaviour when you specify the number of inches as well, and it's not a rule - more of a guideline. You will hear people say "five feet ten" as well, just less often.
If a measure is just being stated, not applied to anything, you can in general uses foot or feet and no-one will mind either way, especially if there's no inches to go with it:
"How tall was he?"
"About five foot"
"How long should this plank be?"
You can think of it in terms of there being different implied words around the simple statement of the measurement, but I suspect that would be more confusing than enlightening.
In the particular dialect I naturally speak, there's at least one interesting exception to this "either way" thing. If you're talking about distances between objects, you would always use feet.
"So, how far is it until the next waypoint?"
"About 50 feet."
However, in other British English dialects, it would be the other way around - you would use foot in that situation.
Basically, the biggest take-home for this question should be: this is a horrible quagmire of expectations that are confusing even if you just look at one dialect, and if you start worrying about more than one dialect you will realise that you can never get it right according to all native English speakers.