I just posted this in another thread, so instead of re-typing, I'll copy/paste it here......
Here is a good rule of thumb, if you want to ensure fast, healthy growth.....don't let your plants get bigger than their pots. For example, if your pot is 6" x 6", don't let your plants get much larger than 6" x 6". Yes, there's some leeway there, but try not to let 'em get much more than 25% the pots volume, which in the case, the plant would occupy 8" x 8" of space. They start getting restricted, at that point, and will start stretching upwards, rather than outwards. Ever notice that you never see a 12" wide,(untrained) bushy plant, in a 6" pot? That's because the foliage won't extend much further than the outer edges of the rootmass. When you start getting to the bigger pots, it's not so much of an issue, but still applies. What's going on underground, determines what's going on above ground, directly. IMO, it's never too early to transplant, unless you haven't got the watering/drainage down to a science, yet. The only risk, is the soil staying too wet, for too long(overwatering). Other than that, it's always a good idea to go big, when possible.