Reality check
So there's no hard limit caused by the limited dialogue window space or other reason?
I'm guessing a village could in theory have 6 purchase options (small/average/big - male/female).
I'm not sure if I've ever come across a settlement that sold more than one type of animal, if it is possible then all the purchase options might not fit in the dialogue window.
If I recall correctly the limit of dialogue options in case of animal purchases is 5 entries, but it's not intended that every animal of every different size is a purchase option. For sure you'll find it possible to buy several different types of animals, despite of number of entries being what they are.
In next version, and NPC decisions matter even more as they want to keep certain number of animals for themselves and even if they were several sized cows and pigs, you may notice that only one purchase option for each is available. Despite of many more entries being available for the dialogue list.