Certified GKH Gardening Expert
When deciding how large of a tree to purchase, it is really a personal choice that is made after understanding the pros and cons of a younger (smaller) tree versus an older (larger) tree.
Smaller containers will have younger trees that are less expensive. Younger trees also have a better chance of surviving the shock of being transplanted. But they will take a bit longer to reach mature size.
Trees in larger containers are older. The look more impressive when first planted but they are more perceptible to shock. The older a plant, the more likely it is to go into shock when transplanted. Older trees are also more expensive because they need more care from the nursery.
I personally opt for younger trees. They catch up in size quickly and you are less likely to lose them to shock. But you should look at your own circumstances to decide what is best for you.