Well at a minumum they should have 10" inches. 5" from stalk to rim of container. This is a setup where the containers are 9" in size. You can try and go 8" but make sure you give them space or you will have scrawny plants. If you can veg for 4-6 weeks, you want to give the plant a chance to sexually mature on its own. If you cut the light back before any preflowers develop the plant will still have to mature but doing so in less light. It wont speed things up. The easiest route to a short flowering period is turning the lights to 12/12 when they plants tell you their ready. I got all this from the growers bible, its sounds like good advice and Im going to go by it in my grow. I also hope to flower in the next week or two.