Your crepe myrtles will start leafing out with new growth in a few more weeks so this is the time to do any pruning you might need.
Light pruning will help to keep a nice classic tree form as well as promote stronger blooms this summer. Ideally, you want about 3-5 stems forming the trunk, and the canopy height should equal the height of the trunk. If the canopy gets too big your tree might look more like a shrub and could get top heavy and bend over from the weight of the blooms.
A hard pruning is also an option if you need to control the size of your crepe myrtle tree. This pruning strategy is not ideal for the long term however, so you might want to consider moving the tree and replacing it with a tree that is adequate for the space.