Leaf lettuce is good to eat once it is big enough to pick. You can gently remove the tender leaves when they get about half the lenght of your hand, You can pull some of the plants at this time to thin them out, or just pluck the leaves off. Later when it gets as big as your hand you can cut them. When you cut the stem, what would be the core on a head of lettuce, the plant will not grow anymore. So plucking or cutting the leaves off is a way to keep the lettuce producing for a long time. Thinning out the bigger ones will achieve the same thing and prolong the crop. The flavor of fresh baby lettuce is much milder than the stuff you get at market or in the store so use a lighter dressing. Enjoy