Eggplants come in many sizes and shapes so I can't tell how far along yours are. As long as they are shiny they are fine. When that gloss starts to fade, pick them asap.

You can certainly pick them now if you wish but to answer your original question as farmerdill said, we would need to see a picture and know the variety.

The main penalty of letting eggplants stay on the vine to long is that the seeds develop, and seedy eggplants aren't as nice to eat. Pick one and look at the seeds. If they are getting big, then pick the rest. If not, then leave the others on.