Sicilian?

If these are normal eggplants, you can harvest at any immature sage that is large enough for your use. A mature eggplant is not Palatable. Pick them before they lose their gloss.

@zeedman Zone 5 Wisconsin Sorry, yes Sicilian Eggplants. They are round eggplants.

I concur with @farmerdill's advice. As long as the skin of the eggplant is shiny & feels soft (or 'tacky") to the touch, it can be harvested. When the skin loses its shine & becomes leathery, the seeds within will have hardened, and the flesh will toughen. So it is best not to wait too long; better to pick a little small, than to pick large & unpalatable.