There's only one size as far as I'm aware, although the size varies based on your race. A taru chocobo is smaller than a hume's, for example.
The only way to get a colored chocobo is to raise it yourself. You need to get a chocobo egg from one of the various sources. I believe ISNMs were the best source for a colored chocobo. They come in yellow, red, green, blue, and purple, as far as I know. I have a purple one that I got way back in the day. It takes a fair bit of work to raise them though, and if you slack you can end up with a pretty slow one. Mine is much slower than the rented chocobos.
Edited, May 15th 2012 8:13am by Pergatory
____________________________Pergatory (Asura, Kupo LS) WHM99 ~ SMN99 ~ DRK99 "If you want to know me, look inside your heart." -Lao Tzu