White Out / White Ball / Schneeball only gets about 24x24". There aren't many white dwarfs of there.

Bobo isn't necessarily (very) small, but it blooms on new and old wood so you could prune it down low every spring and get many reliable white blooms.

Its for a fairly narrow border by the driveway and Im concerned about running over anything that spreads too wide. Its also fairly shady so a lot of plants wont work there; so I wanted to alternate astilbes and hydrangeas. i think white out may work!

Yep, and the shadier the better for White Out. The blooms will begin to scorch even in a few hours of sun (after the first week of being open). The leaves will also be more attractive in the shade. Not exceptionally winter-hardy, so if you get some snow or leaves, throw some on over the back seasons.

Wedding Gown (double white lacecap) is also supposed to be dwarf-ish in stature with a slightly spreading habit. It could be trimmed here and there as the growing season progresses. WG is becoming more readily available at nurseries this year.