Usually at the tire manufacturers web site or catalogue you will find all the technical information about each size such as section width, tread depth, etc., including overall diameter.
Problem I have noticed is different MFG'S even with the exact size have different specs. Some are off quite a bit. If you log onto TIRERACK.COM and simply plug in your size and then hit specs. You will see exact size tires vary from mfg to mfg. Also note how rims size changes measurements.
I used to build bikes and the tire sizes seemed to have nothing to do with what the mfg's had on their sidewalls. If you want to be very accurate check with the mfg's specs.
Now if I can just stop playing with that calculator on the web site. Great info.
tirerack.com has specs on each tire they sell, including overall diameter. works awesome when your deciding on what tires you want on the front and rear without getting something totally off