All I know is my own experience, but with all three of our males we waited
until they were over six months old and had fully dropped and developed
testacles before we fixed them, and they are all quite large--like over four
pounds.  Of the other fuzzies we've run into, the ones who were fixed young
are much smaller than our boys, while the ones who were not are closer to
their size.
 
Adrienne, Barry
Magnum, Faux-Pas, Snafu, Carlin
[Posted in FML issue 1468]