6 on 5.5" is the bolt pattern.
Stock Xterras come with 40mm offset wheels. I think by 2003 the 15" had been phased out, but it will fit regardless. 15x7, 16x7, and 17x8 have all been OEM wheel sizes.
Tip: In the back of the owner's manual in the tire/wheel section it actually lists all the factory sizes, with wheel size and offset.