Crepe Myrtles are generally very easy to transplant. If it was just planted this year, it should be even easier. If the city planted it, I assume it isn't too big yet. When you dig it up, try not to disturb the root ball too much. Come out about a foot from the trunk and cut straight down with the shovel first, then remove the dirt outside that cut. If you just stick the shovel in the dirt and start prying up on the roots, you can snap or damage too many. When you plant it, keep it watered daily for a few weeks, then start cutting back. Use the finger test to make sure you aren't over/under watering. Stick your finger in the dirt next to the roots, if it's dry- water.
Crepes usually don't get too big. They vary from 20-40 feet mature size depending on the variety.