No, they are not completely full size at 20 weeks; they'll continue to fill out, but you will be able to tell who is stocky as they should be and have a good handle on how their color will be by then. All four of my pullets were laying by 21 weeks old, the earliest at 18 weeks. After they began laying, they really started filling out. The main thing about color is that if they have very little at the beginning, they will always be lightly colored, compared to others. The ones with a good amount of smut as chicks will be the better colored ones. Some of this depends on the group you have. Some will stand out early on. Isaac did. I called him the Rockstar at four weeks old because I saw huge potential in him, thanks to the mentoring of seriousbill. I narrowed it down to three finalists among the cockerels at 15 weeks old and I just knew that Ike was the ONE by then.
I usually give my chicks scrambled or hardboiled egg several times a week. It's fabulous fresh protein for them.