Here, in the U.S., they're always called Egg Plants.
We harvest them when we notice they've stopped growing. Depending on variety, they'll be about 6 to 8 inches in length and perhaps 4 to 5 inches across. The skins should be shiny and taught... no wrinkling.
Be sure to harvest them with a pair of shears leaving maybe 2 to 4 inches of the stem.
To assure correct harvest time, there should be several sizeable but not hardening seeds when you cut it open.
Look up a good ratatouille recipe!