Bring some current photos of the planting site with you. These will help your supplier in determining which items will fit within the context of your planting site. It makes it cheaper for the garden center to buy due to greatly reduced shipping costs. It makes for easier loading in the trunk of a car or the back of a small pickup truck. Anyone can handle the tree because the weight is limited.
Planting is easy for the homeowner with undersized nursery stock.
Yes, you can put it in to a small car or truck for transport home, and it's profitable for the garden center. Here are a few examples of what the American Standard for Nursery Stock requires all nurseries and garden centers to supply it's customers. A 2 caliper shade tree should be between 12 to 14 feet tall. Next the root ball must be of a certain size based on the size of the tree or shrub. Normally weight relates to ball size.