Acoustic Treatment and Design for Recording Studios and Listening Rooms
He had some preplanned rooms somewhere on his site or on his forum that follow Aerius' suggestion -- you'd be surprised by how little room you lose -- but I can't find them right now. It's not a continuous taper, you can taper in sort of a terraced approach, ends up having a sawtooth look to it. I mean, you're going to lose a little bit of space anyway, since garage walls are thinner than house walls -- you're going to have to build them out into the room anyway. Unless you completely demolish the room and start over, which shouldn't be necessary.
Also -- as much as he seems to know about room acoustics, he doesn't seem to know (or care to know) anything about soundproofing, so don't go to him for advice in that area. He specializes in minimizing reflections, mostly bass reflections.
Another suggestion: humidity control. And I don't mean just get a dehumidifier -- there's a way to design a room to let it "breathe", so you don't have airflow nulls where humidity will accumulate -- make sure that whomever you hire knows this already.