We harvest ours when it's about the size of a cantelope. The head will remain firm. Keep an eye out for lots of holes in the leaves. Cabbage moths will lay their eggs and they will eat into the cabbage.

Here is a link that might be useful: My Garden Blog

It depends on what kind of cabbage you're growing. A two pound head at maturity is going to be a lot smaller than a variety that gets to be 10 pounds. When you gently squeeze the head it should be firm, not soft. If it's soft it's still forming.

Thanks, I think it is probably time to harvest a couple of them, they are firm. And I do have some holes in some of the leaves. Maybe the bugs left me enough to eat!