This time I waited until there were lots of leaves at least a foot long just so I'd have a big batch to work with. But I don't think it really matters. When a leaf is big enough to see it's big enough to eat. LOL. I'm sure younger leaves would be more tender and cook more quickly.
I cut off all the lower leaves and left the crown of smaller leaves so the plants will continue growing and producing through the summer. Last year they started to bolt in early fall.
A few years ago I had plants that survived through the winter, even in temps in the teens. Didn't phase the red Russian kale, which is cold hardy.