I just had 10' x 10.6' stalls built. The contractor used oak boards everywhere except the bottom boards - they're pressure treated because of contact with urine. The posts are pressure treated as well. We went with double dutch doors which are easier to build and cheaper.
As far as size, I thought my gelding would seem squished but at 15.2 hands he's been laying down, rolling, and just fine in a stall that size. Granted I would love to have gone bigger, but sometimes you have to work with the space you're given :)