I side with sir Visey ..... i use a 5 gal primary. and the blow-off tube I use it ....1 inch ..O.D.? give or take...buuuuuttttt. I rap the very end, like the last 1/4 inch teflon tape ( like what plumers use) and it fits really tight in my carboy! ( I am not sure what a 6.5 gal. carboy would use) anyways the tube sould be just inside the neck of the bottle!
See you at the Bottem!
sorry read it wrong... the end not in the beer needs to be in some kind of water to keep "bad" air out!