I would just go with whatever feels "right" to you ... cos you're going to get a boat-load of different answers depending on what everybodys views are.
For us, Daddy and Linus both were neutered at approx 5-6 months but that's because I couldn't take the humping any longer!!! Daddy just turned 2 and still seems to be getting broader across the chest. Linus is only just 1yrs old, and has stopped growing up and is visibly filling out now all over. Who knows if I've stopped them being as big as they could have been ... I just feel happier having them neutered.