Answer: Osage crape myrtle will grow 15-20' tall although you can probably keep it at about 12-15' with annual pruning. I don't think you can keep it less than 6' tall without sacrificing flowering and compromising its health. If you want a smaller crape myrtle, why not replace it with a semi-dwarf or a dwarf plant? Petite Pinkie has the same color flower and grows to a mature height of 4-6'; likewise for Petite Plum.
Best wishes with your crape myrtle.
Q&A Library Searching Tips