I've got 13's to go on mine but am using 175/50r13, it is only a tiny bit larger than the 165/7010#'s it runs today so i am hopeful it will fit.
My car does have flared front and rear arches though.
I think it'll look quite cool when done, you can also use 12's, whcih is the way I'm going on my more stock spec N600.
There are pics on this link of both the N600 and z600 with 12's and 13's.
http://www.auto-otaku.com/home/2008/02/ ... s-mee.html
Getting the 175?50r13 tyres may be an issue for you guys in the US but they are a size used on the mini sportpack cars in europe and as such you can buy several brands inc Yokohama A048R's whcih is what i'ev got.