Oh, I forgot to mention, they're thornless; I don't know if that is helpful or not...

Well, I would say plant them 3 to 4 feet apart. A trellis structure is fine. IMHO, the "best" way to control their spread is to put them in a separate bed surrounded on all sides by turf, so that any suckers that come up in the lawn are cut. Alternatively, I suppose some type of physical barrier similar to the "bamboo barrier" used to control the spread of bamboo would work. In your zone, a third alternative would probably be to grow them in very large containers. In an open garden situation, yes, they will spread if you allow them, but you can remove the ones that come up beyond their designated space a few times a season as an alternate means of control. Since they're a thornless type, that is a lot less painful than if they were thorny.

Height is hard to say, because it varies so much by variety -- some of the upright kinds can get 8 to 12 feet tall in the right climate, while some of the trailers won't get up off of the ground to speak of without help of a trellis structure. Yield, too, is kind of hard to say, because there are just so many variables, including berry size. My patch that is roughly 8 x 20 will give me maybe 10-12-15 quarts in a good year after bird competition, which generally isn't bad enough to merit covering them since the birds seem to prefer other fruits first.

Thornless varieties don't produce suckers. They propagate by layering. Wherever the canes touch the ground, they will grow roots. So it is important to support canes on a structure. Overall thornless blackberries are much easier to control than thorny suckering blackberries.
Olga

Okay, thank you Olga. Dennimi, that was really helpful as well and good to know. I'll pray that these things don't try to take over, and as a precaution, put them in a dedicated area just in case.

Blackberry is the easiest fruit tree to maintain. I started with one tree and now has three pushes. They produce abundantly so far we had over 2 bushels this year, It is very irresistible to disease and can protect itself from the rabbits and the deer by inflicting pain by its thorns.They produce suckers and I am planning to replant these suckers along the fence. My grand kids have a feast with Blackberry. But be careful working with them wear gloves.

FP, are you speaking of the blackberry bushes or blackberry vines? I have blackberry vines and they're thornless...