Canopy trees are those which grow the tallest and have the potential to overtop other plants. Understory trees are smaller, and are (usually) under other trees' leaves. So understory trees have to be more shade tolerant. But it's important to distinguish when you're doing this outside if you're looking at which level the trees are in at the present time, vs the potential of the tree to grow above others. For instance, ironwood is almost always an understory tree, but beech can be a canopy tree. But in some places, you might only find beeches that are uner 20 feet tall, but oaks in the same place are 60 fet or taller. So in this location, beech is an understory tree, but it's capable of being a canopy tree in other places. And sometimes it depends not only on the species, but the location. The same species in eastern states may be an understory tree, but out west, unless it's surrounded by pines, it may be a canopy tree.
Maples can vary by species. A sugar maple can be overstory, but striped maple never is.
The height will be related to the dbh, but isn't always dependent on it. You could have a tall spindly tree, or a fat tree where the top has been broken off.
I would classify them like this based on their capability for growth, but your results might be different depending on the actual height of the trees.
Ironwood - U
Maple - depends on species
Red Oak - C
Beech - C
Basswood - C
Ash - C
Box Elder - U
Cottonwood - depends on species and location
Cherry - usually C, but some species are U
Buckthorn - U
