Beautiful plant. Never had one but wow, it's a room-changer.
What purpleinopp said:)
As far as size, I'd aim for about 1-2" wider than the root ball.
As you fill with soil make sure to tamp down that outer loose new soil. I do it again when I'm done.
For a plant that size and mass, if the new soil is loose it can shift a lot when you water.
To be on the safe side you could even cage it with 3 or 4 bamboo stakes off the perimeter where it won't harm any roots, tied gently. I get them for about 10 cents. Leave them for a few weeks and remove once the new roots lock in.
