That's where I'd fall short on advice. I wouldn't know what size pot to use. I only have one in a pot here and it is "large" but have never measured width, depth or height. I'll do that when I go out but some clematis roots are huge (or, in the right conditions can certainly grow huge) so I think the larger pots would be better but I still don't know what size!
Definitely buy the composted manure!
Roguchi, from a friend who delivered it years ago:
Blue Light, upon arrival - grab your tissues:
Blue Light, from a different source but the same year as the abomination above and my fault for letting it get so potbound!
Finally, a bloom on Blue Light:
