I use my 3 gallon BB for 2.5 gallon batches all the time, and NEVER have needed a blowoff tube. I pre-measured and marked my Better bottle, and you can see here where the 2.5 gallon mark is. If you can't tell it is right where the krausen begins.
I've had some mighty fermentations, and never had a blowoff in it (actually I've only ever had one blowoff in any beer batch and that was a bucket.)
But obviously if you feel you need one go for it...but I've not seen the need. And I've done some pretty big beers in mine.
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Revvy's one of the cool reverends. He has a Harley and a t-shirt that says on the back "If you can read this, the bitch was Raptured. - Madman
I gotta tell ya, just between us girls, that Revvy is HOT. Very tall, gorgeous grey hair and a terrific smile. He's very good looking in person, with a charismatic personality... he drives like a ****ing maniac! - YooperBrew