Breeding for comb size?
I don't know if anyone has tried to do this in BYC. It will be an interesting experiment for you regardless. Much has been accomplished in the way of breeding by selecting for a specific trait. Wish I knew more about the genes that cover Comb and Wattle size. You should be able to report back here if it's a dominant trait or not when you hatch the eggs fertilized by your little rooster.
Keep accurate measurements of comb sizes and select the smallest combed females and breed them back to their father unless you find a son with yet a smaller comb, then choose him instead. You should see a notable reduction in size after a few generations, providing it's a dominant trait. Yes, this is inbreeding at its worst and you may bring forth a host of unwanted recessive characteristics but it's the quickest way to achieve your goal.
If it's a polygenic trait or a recessive trait, however, reduced comb size can still be accomplished but it may take many more generations.