What size pots?

Hello, I’ve just bought some climbing roses that grow between 1.5 to 2.5m and about 2m across. I can’t for the life of my dig into my borders - they are completely solid clay even when wet. It does say on the label that are for city gardens and patios so I am assuming they can be put in pots - but does anyone have any idea what size to buy? I’m from the UK in case anyone has ideas as to what pots to buy. Thank you in advance 🤞