You can actually flower them right from the start. There are several threads around here on 12/12 from seed. I believe the plants will stay fairly small (under 2 ft).
I've even seen a guy who grew a "bonsai" cannabis plant. He let it grow for like a year before he actually harvested it. There's a video on Youtube - search for "bonsai cannabis" and look for videos from a guy named Jason0930. Pretty crazy stuff.
You can also prune your plants to keep them smaller, or buy some of those "lowryder" or "lowlife" seeds from one of the seed banks. They stay pretty small.
