How do you judge the size of a pot to help the trunk on a pre-bonsai grow, when you can not plant it in the grown. Purchase pre-bonsai in bulb pans, and what to let the trunks grow and start shaping as needed. Trunks are from 1 to 2 inches. thank you

Ideal are shallow pots. And do not make them too big. The plant needs to be able to 'control' the moisture levels in the pot. I would say.. Professional tree growers go up one size once the grootball is stable i the current pot. This means moving them from pot to pot two or three times are year. That appearently is the ooptimal way to get maximum growth. I would probably put them in pots twice the size of the previous. But pot growing is a thing initself..

Moving form pot to pot at least 2 times a year? Do you also trim the roots?

I grow mine out in the ground. I was just saying that professional tree nurseries do this. And no. The of course do not trim the roots. They are in the tree business, not bonsai