Hi there i am looking at buying a rocket bunny kit for my 2004 350z st coupe and was wondering the best rims size and offset would be best to fill the fender gaps

 

This...


and this...
https://my350z.com/forum/tire_rim_calculator.php
will tell you what you need.

Don't worry about diameters, you can figure that out whether you go 17s or 22s. (or any in between.)

EDIT: I just re-read this and realized it may come across as a wiseguy answer but should tell you that I'm dead serious. Maybe a current RB-equipped Z owner could tell you this with some authority but really the main thing you should trust is yourself and the empirical evidence (measurements) you gather. Using the wheel offset calculator gives you the other tool you need.

 

front 19x10.5 offset -15 & 19x12.5 offset -55

go around those numbers