Loch Ness Monster (Blackberry)

I have a Loch Ness Blackberry plant over from my ordering of fruit bushes and have read that it can be grown in a large tub for patios.

How large is large do you think? I understand this has an upright habit rather than scrambling like its wild origin. It would be nice to have some breakfast fruit in the yard without having to go down the allotment for it.