Ok - that helps a lot! If you're looking for green-only, you can put a pair of 300-400 mw units in there and they'll look great. (Why have just one, when you can put two in there - still running off a single controller - and have the X signal reversed on one so they'll mirror each other.) Also, the lasers will be quite affordable at that power level.
If you want to go full RGB, you'd be fine at 1 watt. More than that is overkill for such a small venue.
That's my opinion anyway! Also, glad to hear that you're aware of the CDRH requirements.
Adam