When do you pick onions?

Ok, I planted my first batch of onions...the red ones and they are up and very tall. I noticed that one had a little sprout looking things at the top of the green, so I thought that they were ready to pull. When I pulled it up though, it was just the size of a green onion like you get at the store. I want my onions to get big, so that we can put slices on hamburgers. How will I know when to pick them?