OK, how to make your bonsai bigger might seem a little counter intuitive. After all isn't the whole point of Bonsai to have small trees?
Well, yeah, sure that is true, but already Bonsai come in a variety of sizes and I think its safe to say that all of them are much smaller than they would be if left to their own preferences. So If you want your tree to be bigger, go for it. It's your tree.
Over the years a few customers have wanted to know what to do to get their trees to grow faster - and bigger. So, can it be done? Absolutely.
In fact, it's pretty easy. Move your bonsai to a bigger pot or put it in the ground.
By bigger pot I don't mean a bigger bonsai pot, I mean a regular plant pot that is deeper and wider than what your tree is in now. The "deeper" is an important part of the equation because deeper/taller pots drain better than shallow pots and if you want your plant to sit in an over sized pot one thing that is absolutely critical to be to sure that it drains very well so that your tree does not become waterlogged.
Before you remove your bonsai, check the underside of the pot to be sure that it isn't wired in. Once you've done that remove your tree and carefully pot it up with a lightweight potting mixture. Avoid heavy soil like garden soil or triple mix or anything like that. Once your bonsai is in the new pot, set it away from the light for a day or two until the leaves are not drooping and the tree looks like it's forgiven you for the move.
If you can put your bonsai outside for the summer- do it. You want it to grow and it will do so much faster outside. Just be careful to only gradually put it in the sun so you don't burn it. Next step, fertilize at full strength every 2 weeks until November. (you can use a standard tropical plant food). Unless a branch or shoot is growing in a totally ugly direction and leaving it on means that eventually when you finally have to remove it, you're ending up with a big ugly scar, do not trim or prune your bonsai. Will it lose much of it's shape? Probably, but you'll need to deal with that later. Right now, you just want to put on growth.
Be prepared to leave your bonsai in that oversized pot for a year or two. Once the size is more to your liking, you'll shift it back to a smaller pot after some styling and root pruning, and later you'll tackle the foliage trimming and styling, but you don't have to worry about that one for a while.
Your other option with indoor trees is to put them into the ground for the summer. I've done this many times with Chinese Elm and Rosemary to get a growth push over the summer.
1- set them still in their pot outside for a few days and gradually bring them into the light they'll be growing in once you put them into the ground.
2- Plant them in GOOD soil (vegetable gardens are good) and at least 5 hours of direct sun.
3- Be darned sure to dig them up before the frost hits.
4- Do not put them directly back into a bonsai pot. Combining the shock of being moved twice in one year with cutting back the roots enough to go back into a bonsai pot could be too much stress for a tree AND its entirely possible that you won't put on enough growth in one year to satisfy that Bigger Bonsai image you have in your hear. Also, a lot of garden soil will stay attached to the roots and you dont wnat to ever put garden soil in a bonsai pot. Just use a lightweight potting soil and winter your bonsai safe inside in a regular, if somewhat over sized plant pot.
5- Once you decide you're happy with the new size, winter your bonsai in the regular plant pot and in the spring, you'll do the root pruning to get it back into an appropriately sized pot- probably bigger than the one you already have.
6- Once your bonsai has fully recovered from the root pruning and return to a bonsai pot, you'll be able to start to tackle the challenge ( and I use that word in the bestest, funnest way possible) of restyling your tree to once again resume it's elegant bonsai lines.