There are a couple factors to consider when deciding to pick eggplant:
Size – I like to pick my eggplant when they get to be a little bigger than my hand. As with other vegetables, it is better to pick eggplant when they are young and tender. Picking them early will also encourage the plant to grow more, and will help to extend the growing season. The eggplant does not have to be those behemoths you see at the supermarket.
Shine – An eggplant is ready for the picking when it gets a nice reflective sheen to it. Grasp an eggplant you might think is ready, and carefully hold it up to the light – if it appears to be very shiny, it is ready.