is there any benefit to having bigger size rims in back than in front? the HR nismos have 18's in front and 19's in back so i was just wondering if this affects anything for better or for worse ?

 

Not that I can see.

 

It's fun when you go to the tire store 'cuz it freaks 'em out.
My guess would be better handling characteristics.

 

Strictly for visual effect. If you want performance and handling, you'd go with a smaller diameter, wider wheels.

Look at the trend in cars over the last twenty years. My 280Z had 14" wheels, my 300ZX had 16" wheels, the 350 came with 18" and now the 370s (and Nismo) coming with 18/19 and the GTR with 20".