That is big enough, they are manageable in the ground as well if you know the rules of their growth I spoke of above.I've read about the 1 year, 2 year cane thing, but last year was the first year I've attempted to grow blackberries, so I don't know much. I've got them in 15 gallon pots.
The rules are all in that above post, we went from wtf where are the berries to holy shit what to do with all these berries. You will kill it if you start keeping last years vines. Ours are in the ground and send out 5-8 20 foot vines each year, that could fill so much fence and make so many berries. Some people use wire trellis as well, they will climb anything.
I'm gonna fire up the I pad and try to get you a few pictures haha