Georgen is a beast man and he is absolutely dead on. With that said though, if you are starting out again and after 90 days of p90 already; I would go with heavier weights. Normally and this works for me, is to do 3-4 sets with 4-8 reps. I'd do that for a few weeks and then switch it up to mid-high volume lifting for a bit and then back.
A few things aboug going heavier weights to remember:
1) Have a good spotter
2) CONTROL the weight and don't let it dictate your motion. Maintain your form
3) Check your ego at the door. Meaning, don't try to push yourself into trying to do weights that the guy next to you can do. If you do attempt it, remember step 2.
4) For size training, sqeeze and flext at the peak of each rep.
5) To gain size, your body have to be in anabolic state (if you do not know what this is, read and learn a little bit more on dieting - dieting is key to size gain and lost).