My favorite understory tree right now is Forest Pansy. It is a red leafed redbud, sp. cercis. It has outstanding fall color, heart shaped leaves, and a nice winter form. Best of all it should stay under 25'. Or how about a magnolia? There are many beautiful varieties that will also not get too big.

Maybe some fruiting shrubs like blueberries for instance.
Also, spice bush and native viburnums to name a few. Perhaps a witch hazel? You can also look up shrubs and trees that have the ability to grow taller than your max. height but that can survive a harsh pruning.

Have thought of magnolias as upperstory trees, and will look at the shorter varieties. Also added some evergreen and deciduous viburnums to the list. This area is all full sun and will save the forest pansy for shade of garage. Has anyone grown the forest pansy from seed? Regular redbud seeds have grown well for me. Also thought I could plant some Amur maackia. Recall someone sayiung that redbuds could be planted four feet from each other in all directions in clumps of 15-20. Sounds cruel. CellaJane