I would have said that a 9cm pot was a 'reasonable' size not small but I don't think I'd be planting out now although it will depend on where you are in the UK. Temps here in the Midlands have really dropped in the past few days but it might still be quite mild where you are. That said, Stipa G is fully hardy in the UK, so by not planting out you are just removing the possibility of the plants sitting in (possibly) waterlogged soil over the winter.

Thanks. It's still mild here, (London), but wet and my soil is heavy - I think I'll plant one out and overwinter the others in pots by way of experiment...

You could spend the time improving the drainage where you intend planting them - they are original Southern European so would do better in free draining soil than in London clay(?)