I have a 1930 Model A and I did a lot of measuring and research before buying the wheels and tires. I went to the BFgoodrich web site and they had a section with all the tire sizes along with the widths and the diamaters of each tire. You will need this information along with the measurements you take off your car to make the right decision. You will also need the frontspace, and backspace measurements on the rims you plan to buy.
I ended up getting the P-255 70 R15 tire on a 15 X 8 inch rim which has a diamater of 28 inches for the back. Up front I put the 195 70R14 tire on a 14 X 6 inch rim which has a diamater of 24 inches.
This ended up to be the perfect set up for this car as it gave the car just the right stance, looked cool, and still handles well. If you go with a tire diameter difference of more than 4 inches front to rear the car is more difficult to handle on corners.
If you plan to go with the same size tires front and rear the planning is easier but the car doesn't look as cool. I guess it just depends on what you want there.