Size doesnt really matter on cockfighting but rather would be entirely depending on how that rooster had been trained off plus having that good body condition.I dont know on how these cock trainers do train out their cocks but its true that luck would be always take up a huge factor on determining the winner of the said fight.
Game cocks are being treated with regular vitamins and supplements to help its muscle grow especially in the leg area as that was where the blade is attached. With strong legs too, these cocks will improve their leaping ability which is a good advantage. The physical training is done thru fighting with other game cocks but of course, without the metal blade attached to its legs.
I've already witnessed and seen several practices and training related to this many times as it was a common hobby back then during my childhood days in the 90s when there was even a practice right directly on the street.
That's how expensive this kind of activity is for the owners. Here in our country, cockfighting is one of the biggest gambling industry here.