I don't know about yours, but mine are "Boltardy " and it said on the packet they would be ready when they were 2 to 3 ins (50-75mm) in diameter. And a local gardener said anything over golf ball size would be fine.
I've followed this advice and mine have always been splendid although some were smaller due to my impatience !
But of course, yours may be a different type ?

You can try scraping away a little of the soil to see the size, as Hank says anything larger than a golf ball is fine...

I've left them in the ground until late autumn and they have been larger than the recommended 3" but still usable. One guy on the allotment could have played football with his but they would probably were a bit woody.
I grow Cylindrica because as they grow they lift themselves out of the soil and when you cut them the slices are all the same size. Then your choice would not be the diameter but the length and the size of your pan.