What's the best way to measure for tire size.

I have 3\4" clearance to the leaf spring and about the same on the fender side. Drivers side is a little tighter to the fender, I'll move it later when I get back to the rear end. New front end is next.
I was looking over muumuu's car at willow springs...
Muumuu's car is a lot tighter than mine but I have the offset and he doesn't, yet. I think He's using the same size wheels and tires.