Certified GKH Gardening Expert
That size is up to you, and how much work that you are willing to do to keep them alive! Larger pots, say 20 gallons or so, will not require much work. The trees will remain happy in a large container, as long as you scale up over a few years time.
If you would like to keep it in something more like 5 or 10 gallons, then you will need to do some root pruning every 5 years or so. This isn't hard to do, but it does require care. Since you will be cutting large roots, it will be necessary to mitigate any disease before it happens. Fungicides and biofungicides will be a good idea.
Here are some articles that will help: