sparks,
There are any number of sites that will help you select the proper size carb, such as the Holley carb site.
In general you will need the displacement of the engine and the approximate volumetric efficiency at max RPM. The vol eff will probably be around 80%, depending on port and cam limitations.
For an engine in the 250 - 300 ci range that number will probably come out around 350 - 450 cfm @ 1.5 in. hg.
That is a good starting point at least. You can try bigger until you start to get driveability issues.Good luck.
