Sources say it gets 16-20" high, but my plant still is small; the tallest purple spike only reaches 11". I have 2 plants and the largest has spread 12" wide so I know it shouldn't spread anymore (and that's too bad because I'd like more!).
I consider this a delicate flower (although photos on the web make them look like salvia), small and diminutive, which is why I chose to plant it where I could view it from indoors or the patio. It's also part of my butterfly collection of plants -- but I have yet to see any butterflies. They haven't gotten the message yet.