Big as you can get! Minimum 18". And don't forget to water and feed regularly in the Summer.
But be careful, I went to move my Sussex last Spring. The bugger had gone through the bottom of the pot and rooted itself, surprisingly it stayed where it was!
I know everyone has restrictions especially if SWMBO is involved but they really do much better in the ground. Also bine weevil can be a real problem, they seem to love potted plants but won't touch anything in the ground.
Can't be arsed to keep changing this bit, so, drinking some beer and wanting to brew many more!
Sir, you are drunk! Yes madam, and you are ugly, but in the morning I shall be sober! - WSC