Potting a grafted apple (G214)?

Today I grafted 2 rootstocks (both G214) with scion from a very old apple tree at my childhood home. It is likely going to die within the next decade and I want to save a part of it if possible. Assuming that at least 1 of the grafts takes, I want to plant it in a pot as I do not have land of my own and my parents are planning on downsizing in a couple years.

What is a good container size (in terms of depth and diameter or gallon size) to start from? Most of what I've found has suggested sizing up 2-4" from it's starting pot size but since my rootstock came in a plastic bag I can't use that as reference.