As soon as they're big enough to suit you. I personally wait until the larger stalks are at least a foot long -- they're kind of watery and not as flavorful when too small.

When I harvest, I do a total cut down -- pick everything. Then, I throw a little fertilizer on it, and let it grow again. You can do this 4-5 times a season. I've had the same clumps for 20 plus years, so it doesn't seem to slow them down in any way.

When leaf span is bigger than your outstretched fingers of your hand.