When selecting tires with a staggered fit for a custom application you should pay attention to the overall diameter. Most vehicles will use the same size overall diameter on the front as on the rear.
An example is a Mustang that uses a 245/45R17 front (25.7 overall diameter) and a 275/40R17 rear (25.7 overall diameter).
The 40 refers to the percentage of height to width.
The sidewall is 40 percent of the width of the tire which is 275mm wide.
275 X .40 = 110mm tall sidewall
while 45 percent of 245mm will be the same.
We offer specs for most sizes on the specs tab of every tire model on our website.
As an example, click the "Specs" tab here for the BFGoodrich Super Sport A/S.