Hi,
This lilac actually can grow to be 10' to 12'
It all depends on how well you feed it while young.
I used to have one, It was around 1' 1/2 when I bought it.
I watered it well, and make sure it had rich soil manure does the trick, I gave it 15-30-15 vitamines once every two weeks. And it bloomed abundently.
unfortunatly I sold my home and had to leave it behind, becaus it was much too big to take out of the ground ;-(
Hope I was helpful ! Here is a link have a look .

http://canada-gardens.com/2lilac.html

AAAAWWWWWWWWWW! Hoping it was gonna be a 5 footer. Too bad, as lovely as it is, it is just too big for the space I have available. Thanks so much for the imput! Appreciate it...