I don't have any experience with eggplants but I believe you can harvest them at any size. Although you might have to resort to trial and error to find out what size you prefer.

Rodney

I grew those last year. Very good....harvest now!!

With eggplants, it is more about sheen and less about size. You should harvest them before the sheen begins to fade. Early-season eggplants often fade before they reach full-size, and the eggplant in this photo is an example of that (unless my eyes are playing tricks on me). So, pick it now. However, you will find that when the plants get bigger and healthier, the eggplants will grow larger than this before they fade. A normal Rosa Bianca should be the size of a large man's fist.

You can always play it safe and pick them early. No harm in that.

Thanks all for the feedback. Slimy okra the dullness on the first one and the size was throwing me off thanks for explaining! Just had my first seasons harvest of my first vegetable grown from seed; rosabianca!!! So happy! I was gonna have them for dinner but I ended up sauteing one in garlic and oil cause I couldnt wait to try.... best flavor ever!!!