schreff,
I just setup a 20 long, I guess it is about as big as you can get, and still be called a nano. Yesterday I put 2 55W Power Compacts on it, and WOW it is alot of light. It has not cycled yet, but it is not too much light. I have a 4" deep sand bed, and 2 power heads. The mushrooms should adjust to the lighting, if not, then stack some live rock, and give them a little shade.
I have a 7 gal bowfront tank that should finish it's cycle later on this week. It has a 32W PC on it and a 2" deep sand bed, I think that it should be ok too. I have read that several others have 32W PCs on theirs.
Lord of the Reef runs 2 55W PCs on his 20 long + atleast 1 actinic. I can't remember the exact specs at this time. Anyways, good luck with your new tank.