you know what they look like in the store and that is when you want to pick them. if you let them stay for a bit longer the whole head will turn into thousands of little flowers. after you pick the main head let the plant produce side shoots. mine usually produce side shoots for a couple months after the main head is taken. they do get smaller as time passes. you can eat the leaves like any green.
brussels sprouts are the ones that can stay until you pick the whole plant if you want. what i generally do is pick some of the ones off the bottom of the 