Whether you are planting a native or non-native tree, it is best to transplant from a relatively small-sized container. There are several reasons for this. First of all, the longer a plant is in a container, the more time there has been for it to be abused. Container plants require careful irrigation. They can dry out quickly, or remain too moist while in pots. Their growth and general health can be badly impacted, even though they may not exhibit stress at the time of purchase.
A common nursery practice for larger trees is to trim lower branches and stake the young saplings so they have a tree-like form. This removal of low-growing, juvenile branches and staking of the single stem, diminishes the development of a flared, healthy caliper at the base (or crown) of the tree. Trees without this flare are more likely to become uprooted.
Some larger plants have been held in small containers for too long before being potted up. This can result in the roots wrapping around the pot. Again, the container plant may look fine, but as the plant grows, the twisted structure of its roots can result in death. This problem can be detected by a careful evaluation of the root ball at the time of planting.
Larger specimens are also more difficult to transplant. You have to dig a larger hole and carefully decant the specimen and place it in the hole. Soil around the plant must be compressed enough to eliminate large air pockets near the roots, yet not compacted so much that the roots are deprived of water and air. Proper planting is more difficult for larger specimens.
And finally, smaller pots are much less expensive than large boxes. Small trees become established more quickly than trees in larger pots or boxes, so not only is it more economical to start with a smaller size, it only takes a bit more patience and understanding.
So what size is the right size? If we are talking about an oak tree, acorn-size is probably the very best - with a few caveats. The first few years are slow - with little to show. Most people are not quite this patient, and prefer to have visible evidence of their magnificent, future shade tree. Second, acorn sprouts are especially tasty to squirrels and other critters, so without protection, many will disappear. Third, in many locations a small, young sapling may be vulnerable to vandalism or inadvertent damage.