Pruning won't keep it much smaller. It will regrow to the same height and you are likely just to accelerate the sucking.

If you want something that will stay shorter, consider replacing with Gro Low or TIger Eye.

Just my opinion, but I ripped out my gro-lo sumacs. I didn't pay enough attention when the nursery planted them and thought I'd be getting a tree-like shrub with a bit of stature and interest, instead I got a shapeless pile of green leaves. No fall color to speak of, bare branches in winter, barely interesting but not showy flowers in spring. I've seen them grown in other locations so it's not that my specimens were deficient. "Gro-Low" might be fine as background filler in a large garden, but "Tiger Eye" seems more like what I think of when I hear "sumac". I'm sure gardengal is correct, I'm just venting some personal frustration!