What is the highest tires that can be fitted to a 350z?

I know an aspect ratio of 60 is too big, can 55 fit or 50 the largest?

 

The aspect ratio is not a measurement. The aspect ratio represents the tire's sidewall height as a percentage of the tires width (section height divided by section width).

A series 45 tire may be the same height as a 55 series tire depending upon the tires width.

The measurement you are looking for is overall tire diameter. Here is approximate diameters for the 350z:

Front: 25.8" (Overall Diameter)
Rear: 26.7" (Overall Diameter)

 

Ok cool thanks alot Spike