Pullets are sexed, but sexing isn't 100% accurate. Most hatcheries allow for about 10% males incorrectly sexed as females. I think My personal experience would average out to about 1 in 20 as an "oops" boy.
I would select average sized chicks, that's just me. All pullets/hen will lay, some breeds more than others. "Henderson's Chicken Chart" lists basics on most breeds including temperment and how many eggs laid a year. Google it, it's a good one to have on hand.
Oh, and chicken math:
1 coop + chicks+ 1 coop+ chicks+ ooh, we need some of those+ coop+ chicks= 3 coops and houses, spoiled birds, and more on the way!