how do u tell what size carburetorto put on a engine

cubic inch displacement, cam specs and horsepower expected..
or this quick formula:
d = Displacement in Cubic Inches. (d x 2) - 15% = CFM

Carburetor CFM Sizing Formulas - Shop Math
Shop Math for the Auto Hobbyist. . quick and dirty formula for figuring carburetor CFM requirements for a street engine. . This a real ball park formula. .
http://classictruckshop.com/garage/shopmathcfm2.php

Here is a good website:
http://gtsparkplugs.com/CarbCFMCalc.html

RPM plays a big factor in your calculations, use of the CAM plays in the efficiency factor. For standard street driving where you would max out around 3000-3500 RPM, you should use that in the calculations instead of the Redline RPM. For instance, Ford uses a 150CFM on their 200 CID engine. If they were to use the redline RPM, they should have used a 240CFM.

l hope this helps.