no no, Not Wheel size, Tire size. Lots of cars came with 15 inch wheels and didn't get special fenders. Hemi cars had F-60 Tires.
Here's a pic of a Hemi fender.
The part # is listed as a 70-71 Challenger with F-60/15 tire.
One of the guys from creative industries that did the fender modifications on the early cars was here on moparts talking about this recently.

Tav