My "studio" is a 10ft by 12ft spare bedroom. It's full of book shelves, office supplies, and oh BTW, I have some recording equipment in there, too. Would never be able to fit the Seattle Symphony in there, so there's a small restriction to what I can record. I don't think I could get an average size drum set in there, either. Yet another restriction. But, I have never had any desire to record the Seattle Symphony, or any size drum set. I record acoustic guitars and vocals. So, the size of my studio is just perfect, for me. Size is relative.
If you have a choice, I'd personally use the bigger room for tracking and a smaller room for mixing and treat both rooms accordingly with absorption.
If that's not an option, work with what you have.
I'm stuck using a small, (currently untreated) bedroom that is 12'x8'x8' tall. Works good enough though. I mainly do acoustic/electric guitar. Drums and other stuff are done with MIDI.
and we've got a studio construction forum here, too. Do some searches and you'll find what you're looking for. (Which I'll tell you right now will include a bunch of Owens Corning 703 )