I think if we're talking about the heyday for most of the Families it's going to be right at the end of Prohibition. The New York Families did a lot of recruiting for the Castellammarese War and during the period when selling alcohol was illegal and they were making money like it was water. The numbers would have been at their highest then, shrinking due to attrition.

Chicago had higher numbers back then not only for those reasons, but because the Outfit absorbed several other Families. The Outfit absorbed the Chicago Mafia, Chicago Heights and Gary, Indiana. Rockford and Milwaukee also absorbed a lot of Chicago Mafia guys who didn't want to work under Capone. Again though, the membership roster shrunk as members died and weren't replaced, and many of them weren't active so were members in name only. So it's possible that up to the 1950s Chicago had around 300 members. For the Gambinos and Genoveses during the period following Prohibition I've seen numbers as high as 700 or so. I'm not vouching for the accuracy of those numbers, only that the numbers peaked during that period.