Nice man. I wanted to do an underground room but the costs were just to high, and the logistics were terrible (Mountain side bk in the woods 1/4 mile...hard to get a CAT in there, yknow?)
It sounds like you are going to be building a fairly large room, and when it comes to a ceiling the strength you need depends on the depth which you didn't specifiy.... http://waltonfeed.com/old/old/cellar2.html <--That is the plan I wouldve used...I would recommend doing some inner walls out of blocks to support your ceiling.

Best of luck. If you aren't worried about stealth, I would consider just making the ceiling ground level and installing a greenhouse roof. Underground ftw!!! Much more stable temperatures.