We always pick the leaf lettuces as we want them, taking from the largest leaves. Just leave enough for the plant to survive and it won't take long for the small leaves that you leave to grow big enough to replace the ones you took.
As for the size, it's up to you. The bigger the better but we've been known to pick the tiniest little lettuces when we were desperate for a salad.
In Jacksonville the chickweed is growing right now, so do consider adding leaves from those to your salads. We love it and it's an easy to harvest green.
