The stock tire on a 2000 SE is/was a 265/70 R15 BF Goodrich Long Trail. I saw somewhere else where the XE had a 235/70 R15 stock. I know there are some 16 inch rims on Xterras and my 03 S/C SE came with 17 inch stock. The 265/65 R17 is actually a little harder tire to find (and more expensive).
I've seen Xterras on 20 inch rims and they look ridiculous (in my not so humble opinion). Another source for the stock tire size is:
http://www.sizemytires.com/vehicle/2000/nissan/xterraOn a P235/70R15 tire: The 265 refers to the tire width sidewall to sidewall, the 70 refers to the aspect ratio (70% of the 265) and the 15 refers to the rim diameter in inches. Don't know whose brilliant idea it was to include metric and SAE measurements, but it adds to the confusion.
I'd recommend the wider tire for better stability on the road. The narrower tire might perform better in snow.