There were some good columns on room design in Stereophile's Guide To Home Theater (or Ultimate AV if that's what it was called at the time). Some of that info should be on the current website at hometheater.com
Having dimensions which are a simple multiple might cause unpleasant resonances. Plug the dimensions into software that works out that kind of stuff, and see if it's something that has to be addressed or not.
If you want to keep sound from escaping, an easy first step is to use an exterior door with weatherstripping. Beyond that, this site looks helpful: soundproofing.org/infopages/channel.htm
And there's light fixtures and ducting that mustn't rattle or carry sound where it will disturb others.
(edit) This is the subwoofer forum. So, multiple subs are a good idea, both for more and better bass. It wouldn't be a terrible idea to make provisions for wall mounting, since sometimes midway up a wall is a good location. There's different schools of thought. Some say avoid corners because that'll excite room resonances, others say use the corners and enjoy the higher efficiency. I'd be tempted to build columns of subs in the corners or maybe along the front. Four or eight, 12" to 18" depending on budget. Drive with a suitable pro amp, although most likely a couple hundred watts will be plenty given the higher efficiency of an array. Right... there should be an equipment cabinet or room, ventilated outside the media room, for things like fan-cooled amps or media server drive array. I don't know where the best location for players of removable media (like Blu-ray) would be. Built-into a coffee table is tempting; you'd need to allow for cable access. It's awfully nice to have easy access to the rear panels of AV gear. Unless of course you hook it up and never touch it again.
Other thoughts; are you likely to be doing stuff like audio or video capture and editing in this room? Working with a wireless keyboard on your lap isn't ideal. Vinyl playback? How about an isolated cement pedestal complete with sound-tight cover to prevent feedback. Something made from plywood or MDF and filled with rocks and sand would be less permanent.
Moving on to the very superficial... I'd like to have frames on the wall to display film posters. Hinged to make it easy to swap posters. If a full set of leather recliners isn't in the budget, get some genuine fold-up cinema seats; those can often be found dirt cheap. I'm sitting in one right now that the seller claimed from the very cinema where I saw the sneak preview of "Return Of The Living Dead". They tend to be made for sloped floors, but sloping the floor might have an acoustic benefit as well.
