The short answer is you can eat broccoli at any size, leaves and all. Typically, though, most gardeners I know let the crown get six inches or more across before cutting off the top. If the tiny green florets have started to separate it's time to harvest because those will bloom into yellow flowers. Those are edible too, by the way.
If you harvest only the top, or crown or head, I've heard different gardeners call it by different names, and leave the stalk some varieties will keep producing small side shoots as long as you fertilize regularly. These are excellent tasting, just like the tops, only smaller, but again, not every variety produces side shoots. If you have a variety that doesn't, go ahead and pull up and compost the plant after you've harvested the crown. If you don't or you're not sure, wait a while and see if it produces side shoots.
One last note: Use a sharp knife or sheers to cut the stalk. You want a nice clean cut to keep the stalk intact for side heads.