Keep having a look at them and when they are as big as you want them they are ready to harvest.Welcome to GOY by the way any thing you want to know just ask, some ones sure to know the answer.

You can pull them as small as your little finger to eat in salads or lightly steamed and can let them keep growing until they are several inches in diameter - it is your choice really. One way is to wait until there are little finger sized carrots and pull every other one to allow the remaining ones to have room to grow.