The OEM GTR rear 20" ET52 12" rim and fender space can accommodate a 345/30ZR20 tire. If you wish to adjust the stance for a slightly different look, you can add a spacer. For street use, a maximum spacer size of 5mm should not be exceeded to avoid any issues. For track use, you can go up to a 15mm spacer for a more aggressive stance; however, be aware that a 15mm spacer may cause fender rubbing when driving over bumps on the street.

For the front, the OEM 19" ET56 10" rim can accommodate a 285/35ZR19 tire. While a 5mm spacer might also work here to adjust the stance, I have not personally tested a setup with spacer when I had my GTR.

(A note to GTC owners, 285 on front may not fit, the above info is for GTR that has a slightly wider front than the GTC)

thanks for the details.

correct me if im wrong but I would only use a spacer if I install 285/345 and feel that the tires are still a little tucked in?

You are correct. No need for spacer for any of the above. You can use spacers if you only want to modify the look.

A bigger tire is heavier so you are increasing unsprung weight. Adding spacers also adds to the unsprung weight. That's okay for some that are going for a look that pleases them.
I prefer to add lightness as I'm not obsessed with looks.