5x6x9 is not too bad, but inside dim's of 7x8x10 is better, and you can put any dim any way you want - it's still the same modal response, which in this case is as good as you'll get a small room.
It's also big enough for drums, although you might want to allow more on one end for a slat resonator with variable depth - takes some of the flutter echo out, keeps the "sheen" on the cymbals, and, with another bass trap incorporated in whichever corner you can spare, would help tame the kick and low tom leakage.
Here is one thread where one of these resonators is discussed nearly to death...
http://www.recording.org/cgi-local/ubb/ultimatebb.cgi?ubb=get_topic;f=5;t=000309
Remember, the good modal response I mentioned above is based on these being the INSIDE dimensions, so the outside would be 8' wide, which is the legal width for a trailer. If you split the shed into two pieces lengthwise, you could make it 13x7x9 inside, which, figuring 6" walls would make the total outside dims 14x8x10h, which would break down into two halves 7x8x10' tall, and HEAVY if you use mass for soundproofing (don't know any other way) The modal response of this size is still very good -
If you dedicated 2 feet of one end for a combination bass trap/slat resonator (bass trap on bottom) and varied the depth of the traps from (say) 12" to 32", you could tune the slats for specific low mids and the bass trap would work for lows, still leaving you with 7 x 11 feet average inside, and only two pairs of parallel surfaces to deal with.
I don't have the time right now to calculate what all this would weigh, even in two halves - all I can say is, I hope you have a friend with a fork lift or crane -
If you have mor $bux than brawn, here's another idea - these guys make booths that are up to 8-1/2 x 15-1/2, they take down into manageable pallets of parts, and can be put together by two guys in less than half a day - like I said though, $$$$ - also, I don't think they're outdoor rated so you'd have to come up with a "skin" of some kind, not sure what would work there.
http://www.whisperroom.com/WEB_specs/td102144E.htm
This is an 8.5 x 12 size, with "enhanced" isolation - OK, sit down - you sitting? From whisper room's site -
MDL 102144 E ( 8.5 ft x 12 ft ) Door window, lights(2), cable passages(8),
foam sheets(13), ventilation units(4),
Isolation Enhancement Package $15,400
Sooo, since that doesn't include the outdoor "skin", have you considered just buying a small travel trailer and gutting it, adding an under-floor filled with rockwool, furring out the walls and adding Resilient Channel/2 layers of sheet rock, and just dragging the whole thing with you when you leave?
Seriously, might be worth a look when you consider that each HALF of the room I described above will probably weigh around 1500 pounds, assuming 1" of MDF for flooring, 2x4 framing, double sheet rock everywhere inside, 5/8" plywood siding outside, and strong enough framing to be movable, and NOT including whatever framing, etc, for traps/absorbers. Just a rough estimate based on an in-head calc of materials/weights, probably forgot something...
Not to be flip, but have you considered just renting a space? Might be simpler and cheaper... Steve
and am trying to come up with dimensions for it...