Where are you? I can tell you that even in my zone 6, all purple smoke bushes do best as cut back shrubs. They just experience too much die back in most winters. Treated this way, you get a huge flush of great color, but no "smoke." If you decide to try and let it grow, how about 7 feet from the property line?

I leave in Chicago area and I am starting a privacy hedge with shrubs. I got a black lace but I am thinking replacing it with the smock bush and put the black lace somewhere else.
Do you think the smokebush would be a better choice or should I use it just as an accent plant? I wanted it just because of the purple color to match my ninebark.

i have seen them bigger than some modern ranch houses ...

6 feet is possible.. if you rejuvenate prune it every 3 or 4 years...

ken

https://www.google.com/search?q=Royal+purple+smokebush&source=lnms&tbm=isch&sa=X&ved=0CAgQ_AUoAmoVChMIl4qz2Jf5xgIVhmw-Ch18QAxb&biw=918&bih=745&dpr=0.9

A European smoke tree in Walla Walla measured 35' x 35'* in 1993 and one in Spokane was 30' x 35' in 1988.

Walla Walla is in USDA 6 and Spokane is mapped as USDA 5.

*Height x average crown spread

Six feet from the property line should be a fine spacing. The shrubs mentioned here have been growing for decades to become that large.