unused functions aren't included in the final built hex file.
As far as I know, certainly in "normal" tool chains unused stuff is not linked in, but then last time I looked Arduino compiled everything regardless of whether a file had been touched or not. That's not normal so maybe the linking procedure is not as well.
Also I don't know what the granularity is, eg if you use a single function from a module do you get the entire module or just that function.
It would be easy to test but someone here probably knows off the top of their head.
Rob