What a beautiful plant! I don't have much experience in keeping the tree size checked, but from what i read in other forum threads in container gardening, the size will maintain once the roots have been pot bounded (baring in mind the roots will grow out through the bottom holes if the pot is resting directly on the ground). Thereafter, root pruning and changing of soil will maintain the vitality of the plant.

I have cut only the very bottom roots circling the container on my avocado seedlings during up potting, did not disturb the rest of the roots in the soil mass and they didn't wilt, now they are 2 and 1.5 year old seedlings: