Amy One said:
Hi, I have had a Hornbeam, about the size of yours, for over 20 years in a 50 cm pot with no sign of distress. It doesn't grow much. It's probably quite root bound if I take it out but the roots aren't growing through the bottom. It's a good shape and perfectly healthy. I have never pruned it, though Hornbeam can of course be clipped and coppiced and is also recommended for Bonsai, so is probably tolerant of root pruning.