Several bike computer companies have tire circumference data that is generic, not tire model specific. In this chart, the L column is the circumference, presumably L means length of distance for one revolution.
http://www.cateye.com/data/resources..._chart_ENG.pdf

A smooth tire that is good for pavement might be smaller circumference than one with more tread. And of course the knobby ones would be bigger yet, so the table is very generic.

Take the circumference value, divide by pi (3.14159) to get diameter (or height). Then divide by 2 to get radius. Radius would be the distance from the axle to the tire tread, thus that is the number you want.

I am not endorsing Cateye chart, I did a quick google search, that was the first chart I found. I use different brands of computers.