At the restaurant Oscar's in New York City's Waldorf-Astoria, we use young chicken for this dish. I like young chicken for its taste and tenderness, and also for its size. But young chicken, called poussin, is hard to find at the grocery store, so at home I use a small, fresh (not previously frozen), whole chicken (a 3-pound fryer, rather than a larger roaster) with great results. I always look for a free-range chicken, as I think they're more flavorful. You can also use this technique with Cornish game hens.
Choosing the chicken is the easy part; the next step, partially boning the bird, is a little trickier. But don't worry: with a little patience, a big cutting board, and a couple of sharp knives, you'll be successful. The process ensures that the bird flattens easily when the brick is set on top of it. And flattening the chicken is key for two reasons: it exposes the maximum surface area of the chicken skin to the heat of the pan, which ultimately creates the crackling, crisp exterior; and it makes the meat an even thickness so it cooks more evenly—the breast and thigh cook at the same rate and stay juicy, too.
By following the directions in the Photo-essay, you'll wind up with two chicken halves, which I think make very manageable portions (each fits nicely under one brick). But if you decide to cook baby chickens or Cornish game hens under a brick, butterfly them and remove the other bones as described below, but don't split them completely in half. What you'll have is a "spatchcocked" bird (you may have seen this term used in old recipes for small game birds) that's neat enough to cook in one small package.