Just put them in pots that give the roots room to grow, without seeing them it's hard to say, but something like 5" pots should be fine for now. Plant them just deep enough so all the roots are covered. Do you plan to keep them in pots or are you going to plant them out? If keeping them in pots you'll need to repot them in bigger pots as they grow. Keep them well watered but not soggy & give them a bit of feed once they're established & growing well. You don't need to prune them yet.
Thanks, Aquelven. I got two very large pots today and that's almost certainly where they'll stay (can't see dh sorting out the wild patch at the bottom of the garden). Can I put them straight into those or is it better to to enlarge pots gradually?