I've learned a couple of lessons so far about this plant. 1) Keep it quite dry and it won't spread so fast. 2) Plant it somewhere with edging (or even better, in a container) so that it won't spread into your lawn. Deer will eat it, but you'll barely notice. Spotted bellflower can be very invasive—consider yourself warned!
Flowering spikes can get to be about a foot tall, but sometimes they are shorter. All of mine this year are about 6–8 inches tall; last year, closer to a foot. Flowers are large for the size of the plant—about 2 or 3 inches long and an inch wide. The flower's petals are fused into a bell shape, but at the very tips they separate and flare out making a dainty scallop along the bottom edge of the bell.
They are so pretty, and they bloom so well in the shade, that I am willing to put up with the maintenance required. But, I might wind up with a bellflower lawn instead of a grass one at some point. I wish I had known what I've told you when I bought them!