Go to the JB website (jbind.com). There is plenty of literature available on selecting a vacuum pump that is right for you. Try to find a distributor of JB, they may let you test out some vacuum pumps. I'm currently testing a 7 cfm and a 3 cfm.
The most simple way to sum things up is that cfm doesn't mean a whole lot. Once the system reaches a vacuum, which is usually less than a couple minutes on small systems, a 1.5 cfm to a 10 cfm work pretty much the same.
All I work pon is small residential sized systems. I have to play with the ballast within a few minutes to keep it around the 3000-5000 microns.....I'd rather it take a little longer to make sure it's not forming crystals instead of boiling off.