However, I'm thinking about subdividing it down to 9x10 feet (plus closets). The pictures below show a possible layout; the white wall to the bottom is where I'd have to build a new wall to subdivide the room.
My home office is only 8x11.5 and my guest bedroom is 9x11.5. They've both been my bedroom at some point in time.
I've found that shared use open space in the center of the room makes the place feel larger.
While I do have a full size bed centered in the guest room, since your room is dual purpose I'd recommend pushing the bed against the side wall.
It will give you more visual space and you won't feel crowded when you back up your desk chair.
Futons suck. All futons suck. Don't be tempted to get one to serve dual purpose as a couch and bed.
In my home office, I currently have a 2ft deep laminated desk surface that runs the full 11.5' length of the room.
It is supported by a cleat on the back and sides and has a couple plain square table legs on the front that divide it into 3 sections.
The far section has storage bins and a file cabinet under it, the middle section has the desk chair and pc tower, the near section has drawing stuff and a smaller chair.
You could do something like that, then put an oversized couch opposite it. One person could sleep comfortably on an oversized couch instead of having the bed.
You can get 10ft sections of countertop at home improvements stores, although I don't like the incorporated backsplash on a desk.
If you don't need sound isolation, in wall storage built into interior walls is awesome since it doesn't cut into the space of the room and extends it visually.
Since the wall you're building isn't load bearing, you can space the studs however you want. Tall skinny sets of shelves that are taller than the doorways will visually stretch the room vertically and make it feel bigger. You've got 3.5" studs + 1/2" drywall+3/4" finished face less 1/4" back=4.5" deep shelves while only intruding into the room 3/4".
Of course they can't be moved if you decide to rearrange the furniture.