Hi there... I'm new to your forum... I was wondering... I have an 01 Xterra SE and I need new tires. My factory tires are SMALLER than the 02 - 04 size. Is there a differene in the wheel wells? Can I put 265/70/16's on my 01? The factory 01 size looks too small.

With minor trimming of the front mudflap mounting points, yes.
If you need more details, I'd start by searching "trimming" here.