It might sound a bit counter-intuitive, but with bare root trees, smaller is usually better if you want a good root structure.
The concern is not how large the root structure is, but rather, the ratio of root to shoot. By necessity, the nursery will need to trim back the roots a bit when they pull them from the ground. The very fine feeder roots usually break off, and the larger ones are trimmed back so that they'll fit into the bag with the wet sawdust. So a smaller tree will have a greater % of roots in comparison to the rest of the tree. A larger tree, while it may look like it's got a larger root mass, may not have an adequate root system to the bio-mass that remains above ground.
The other thing you'll notice with a bare root tree is that as the roots are trimmed to clean up any broken roots and to help it fit into the bag, they also trim back the branches to balance the root to shoot ratio. So what are you paying extra for? All the pruning just stunts the tree and forces it to use limited
energy to seal off those cuts/wounds, rather than immediately pushing that energy upward toward new growth. Yes, the trunk will be thicker, but that doesn't mean that the tree will grow any faster.
Smaller is better, in most instances. But you're going to have to be attentive to it for the first year until it's established.
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