The general rule is the max size for a food item is the same size as the thickest part of the snake's body. Corns are also known as "red rat snakes" so I am sure a rat of the right size would be fine. I feed my corns multiple mice.

A
-----
1.0 normal BP Nicodemus
1.0 BP Moufasa pos vanilla, fire, cinny
0.4 Cal Kings 3 alb 1 het Dora Queen Ace Pearl
2.0 Alb Corn Bizaar Elixir
0.0.1 Rev Alb Nelsons Oden