Jeremi, The yellow stones ranged from a size 12 upward to about a size 8, the larger they are the lighter (more yellow, less brown) they tend to be.
Not sure it matters though, because nearly every fish I've ever gutted from the Niangua (no matter how "wild looking" and colorful), will have nothing in its stomach but moss and snails. Then again I never kill the bigger ones, so who knows.
How long do you suppose the average stocker has to be in the river before it learns how to eat right ?