Speaking from reading, not experience, during veg, the plant is readying itself for the winter. 12/12 signals that winter's a comin', and the plant goes into overdrive, not only producing flowers, but growing a lot to make room for them. Google "Dr Bud Greengenes" and "Thundurkel." They have threads on icmag and a couple of other sites. They start flowering (in 16oz pots!) when the clone is 3" - 4", and prune it to be a single cola, which can be around 12".
Also, as for rootmass, it seems to have more to do with how well the plant absorbs nutes. There's a thread on RIU by, I think, skunkushybrid, where he actively dampens root growth, but is still able to maintain largish plants by supplying proper nutes.
On the other hand, if you read subcool's organic forum, he recommends sprouting in 1 gallon pots, then transferring to 7 gallon, then 10 gallon, in order to get the most plant for your buck. Maybe not the best for a home grower with limited space...
Hope this helps! I recommend skimming through the site and reading the threads of some of the more experienced growers/more active posters. Skunkushybrid calls himself a new grower, but he's always trying out experiments to test the so-called dogma of weed growing. Heath Robinson has crazy-awesome hydro grows. Garden Knowm literally wrote the book on CFL growing, and has loads of useful threads.
My point is, take what criticism and advice you get here with a big grain of salt. ANYONE can post on these forums, and many people will act like they know what they're talking about, but maybe they don't. The good news is, there are people on this site who have been growing for decades, and are happy to share their knowledge. They get a little annoyed when people ask questions they've already answered in other posts, often in sticky posts. Your best bet is to go the RIU FAQ first. It's a great starting point.
Good luck!
