Why don't I let you measure and do the math? I've kept chicks in the brooder until they are 4 to 5 weeks old and given them 1/2 square feet each. That's when mine get out of the brooder and supplemental heat is no longer provided. Some people keep them in the brooder a lot longer. If your one that keeps them in the brooder longer than that, I'd suggest something more like one square feet per chick. They do grow awfully fast.
I don't know what the options are at the feed store. You can talk to them and see what they say, but you might get somebody that knows what they are talking about and you might not. Chicks often get mixed up at the feed store too, so what is supposed to be in the bins is not what is always in the bin. If we knew what breeds were possibly there, then we might be able to give you some suggestions, but without specific knowledge, it is pure luck. A yellow chick could be a red sex link male, a Leghorn, White Rock, a broiler, or any other white chicken. A barred chicken could be a Barred Rock, black sex link male, or any other barred chicken. A reddish chick could be a red sex link female, or any other red chicken breed. A solid black chick could be a black sex link female, an Australorp, Black Jersy Giant, or any other black chicken. Unless I had some guidance from the people there as to what they might be, I'd probably pick a variety of colors to try to get a better chance at females. It would be a shame if you went with what you thought were barred rock females and wound up with black sex link males. Without knowing any different, I'd probably tend toward the solid black and the red ones, but that is no guarantee.
As far as which specific chick, I'd look for one that is fairly active, not scrunched up and staying still. Obviously avoid any with obvious physical deformaties like crooked toes or ones that hold their head funny, though I've never seen any of these at the feed store. Look at their butt. If it has poop stuck to it, avoid it. It could be pasty butt or it could mean the digesive track is not set up perfectly. Or it could be that it is just dirty. In general, just look for a healthy active chick.
