If you could get a weight on the snake, or post a picture with something in it for size reference, that would help.

But for the most part, I agree with Argentra: if it really is an adult corn, it should be taking adult mice. I did think the 1.5X girth thing was more for growing corns than adults, but Argentra has more experience with corns than I do.

EDIT: To clarify, the 1.5X girth thing probably would be appropriate for a breeding female from what I understood, but since you said you only have this one corn, I assume she is not breeding and only needs a maintenance diet. So hopefully Argentra or someone else with more experience than me will come back and clarify this.