According to my reference, a tree reaches 13 feet tall (3.96 meters) with a single trunk. Lilacs can be shrubs or trees depending on the variety with the trees reaching 25' (7.62 meters). This one is at least 10' (3 meters) tall. However, the single trunk is a tough call. Prune one severely and you will see many new stems spring from the ground.
Photos from my Canon SX620 HS.
@kansuze