I do 5-gal batches in a 6.5 glass carboy, and have done beers up to about 8% ABV, and never had krausen reach the airlock. Been close, though :) For me at any rate, less than 8% beers and 5 gal in a 6.5 carboy, with airlock, works fine. But heed the other's advice on blow-offs. Such a simple...