That's a hard one to pin down exactly because it varies so much. I guess as a very general rule if you are talking about normal sports type models 55-60" span and greater are very likely to run greater than 4s.
Also high performance models like speed models, EDFs and helicopters often use 5 or 6s even at quite small sizes.
Personally I'd get (at least) a 6s charger. That will cover you for just about anything.