You should wait until the base stalk is at least an inch in diameter.

Leeks can be harvested at any time but the thicker the better is my rule.

Pic of average ones

They look pretty small yet. You won't get much meat off of it after you peel the outer layer. When did you plant them? Mature leeks will hold well into the winter if left in the ground but they won't grow much after October.

Late May or early June. I jsut really want to make potato leak soup now and grocery store leeks are not very good

I do agree it is a bit of a waste now.

Not a waste, you still have a couple months of growth yet. Give them some nitrogen every 2 or 3 weeks and keep them watered and you should be able to have the soup this fall. I bet you can find some nice ones at the farmer's market now.

FYI - leek seedlings can be planted out early in the spring. I put mine out in late March here. I start them from seed in early January.

I wouldn’t waste delicately flavoured baby leeks in a robust soup. They're great as leeks vinaigrette. Keep the soup for winter.