If you want to throw a little science at the question you can do the math and find out what your engines CFM requirements are (or should I say, SHOULD be based on this formula). Once you know you can look into what size(s) your carbs should be to best match the number. I've found that it's a pretty good tool. The wildcard is that cfm ratings of carburetors are really not that accurate or consistant accross the different brands, i.e. a Holley 750 may flow more than a Carter 750 or vice versa, etc. The other variable is manifold design and fuel distribution variables between intakes as well as carb designs.

CFM calculator