Crape myrtles require little to no pruning. The type and amount of pruning depends on the desired shape and size of the plant. The time frame to prune is late winter to early spring as they bloom on new growth. They should be pruned to remove unhealthy branches, to remove suckers, and to allow more air movement in the tree canopy.
The best way to maintain a crape myrtle at a particular size is to plant a known cultivar that will mature at the desired height and spread. For more information on pruning see the link from Clemson Cooperative Extension: http://Crape Myrtle Pruning.