for yield, obviously bigger is better, and pruning techniques are essential.
Your going to need more light. Two tubes is nothing, assuming they are T5's thats only ten thousand lumens, that will only stretch like a foot, and if they're not T5's, than its a waste. Growing them four feet will only supply lumens to the top 1/3 of the plant. Look into HID's or more fluoros.
As for sexing your plant, take a clipping and flower it.
I would suggest buying a 250w hps light w ballast and put your mother plant under there, and put the clippings under the fluoros. That 20w bulb is probably completely useless.