Word is the Snow Dazzle is the shortest but all of them are meant to be 6' or less. Plus, pruning helps keep them at the height you want. Sounds like these are going to be perfect for big planters and I'm hoping to use them in the border. Ads say they are really mildew resistant too.

I have enclose a site that might help.
http://www.gardenheights.com/razzle_dazzle_crepemyrtles.html
They are more of a shrub. Great plants