I decided to edit the in-game Wilson Silverstar and add a more realistic approach to it. I based the amount on the gross weight of the trailer in real life and used 1200lbs weight to decide that 48 head of cattle will be about right.
Location: implements -> Animal Transport
Price: $80,000
Brand: Wilson Trailers
Capacity:
Sheep: 38
Pigs: 36
Cattle: 48
Credits:
Lazy E Modding, Giants Software (Base game)