It's all up to you! If the trunk is more or less the size you want it to be for your chosen 'final' style/design, and you don't plan on doing a 'chop' to grow new smaller branches above the chop, etc., then you can put it into a bonsai pot of the size you want to keep it in indefinitely, just doing root pruning when you repot into a fresher soil mix every 1-3 yrs (depending on the species, etc.). It will continue to grow, but much more slowly than if given extra room.

Thanks! I suppose that, as a newcomer to bonsai, I am not really sure of the "normal" proportions that serissa growers go for. I feel that 12 inches is a bit tall for this tree. It has thickened up and filled out a lot this year (I have had it for a little over 3 years), but I think that it might be a bit leggy. I wish that I had a digital camera! Ideas?

Hi - have you looked into some books on bonsai? Because the best way to learn the artistic traditions (and proportions) is by seeing enough well done trees, potted well, that you 'get' it at some point and can see for yourself if the tree seems to overpower the pot in some way - yes, the books will also tell you the 'rules' more specifically, but it helps so much if you can grasp the feeling yourself. Which is why, however, people encourage you to first work with your tree for however long it takes to become a mature bonsai, then look for the right pot, as during the work you may end up chopping the tree down by 70% (happens every day) to grow new, smaller branches, rearrange the existing ones, exchange the apex (top) for another branch entirely, etc. etc. Your tree will just look like 'a tree in a pot' (formal definition or not!) unless it's refined and defined over time, so that the pot becomes integral to the whole thing in the end, and is not just a random choice.