I'd recommend the first Marathon's blue. It just seems right for a keychain rather than Green, though I could see that being good as well. As for size... Well, I'll try for exact numbers, but something like the top of a soda can (2 inches diameter/5 centimeters diameter) seems about right. Maybe a little smaller, but somewhere in there. As for thickness, I don't think a number will do-- just something thick enough that it isn't too heavy but won't fall apart of someone.
On the other hand, I'd much prefer a metal Marathon keychain. =P
