Joe Valachi testified in the 60s that the Genovese family had around 450 men and that Gambino perhaps had the same. Vincent Cafaro testified in the 80s that the Genovese had approximately 400 men.

Then you've got Al D'Arco saying that the Genoveses and Gambinos were capped at (if I remember correctly) 250 men. Which really doesn't mix with what the others were saying.

Either way, I'd guess that the Genovese family was the biggest ever, with peak size being the 30s to the 60s.