I grow Monarch and they get to about 8 - 9" across. To me the key is where I grow them. I have a bed on old pasture, quite a way away from the main garden. The soil is quite sticky, gets easily waterlogged with the run-off from the polytunnel next to it but grows beautiful large celeriac. The "better" more loamy bed in the main garden seems to only produce tennis-ball sized celeriac