We don't distinguish between small and large in cropping; the bed is cleared from one end, and the broadies start to go in as soon as the leeks come out! We like to use small and large leeks for different purposes, and crop about 20 at a time, which we soon get through :P
However, if you leave the little ones in, and if they are an overwintering variety, they will continue to fatten slightly, as they have more room when you've removed the 'neighbours' :)