This depends on how much wine tou are making, The kind of wine your
making and the kind of yeast your making. I have heard of a lot of
people making the Crushedo kit overflowing into the airlock or worse.
Some yeasts foam up alot more than others. I usually use the Red Star
Premier Cuvee or The Cotes Des Blanc for the wines that I make and they
dont foam up very much at all. With these two I would say 1 1/2" is
fine , as for others hopefully someone else will chime in soon.