It depends on how big your Pachira Aquatica has grown. It needs room to grow, but does not need very much space; a smaller pot than many other plants is just fine for this. In general, Pachira Aquatica likes to be somewhat pot-bound. Be sure not to over-water the plant.
There's no predetermined size that is ideal. Rather, check the roots, are they growing out of the plastic container. Like through the holes? If so, your pot's too small. Go bigger.
Like all plants, it depends on the size of the tree. Also, if you want the tree to grow bigger, you'll want to plant it in an oversized pot. Typically, there's never a problem in having a pot that's too big...
