[AlaninCarlisle]

Whatever I do, they never get beyond tennis ball size when grown outside.

Spoke too soon. Either I was just unlucky, or just impatient, when I pulled up a couple but the ones I pulled out today are much bigger. Maybe they put on a spurt in the last few weeks of the growing season as boldondig suggested

[Robster]

I think there is definitely a late spurt of growth with celeriac. I noticed it last year which was my first year with it. Same thing happening this year.

[Annen]

That's a shame, I've just pulled nearly all of mine and by the time they were trimmed they were about the size of walnuts.  I did leave about 4 which were tennis ball sized to grow a bit more, hoping for better.
I will still use the little ones because they are flavour packed and do great in soups.

[compostqueen]

I harvested a lovely celeriac yesterday and added it to the roast veggie tray.   Plenty more of them and most look a good size.