It's a combination of the size of the pond, the type of vegetation on the watershed, the type of soil in the area and how much rain can fall in a 50 to 100 year rain event. Earthen spillways tend to erode and wash out, that is why they are only used for emergencies. Go to your counties NRCS office and talk to their pond guru. They can pull up the watershed around the pond and properly size the overflow pipe. If the 18" pipe was spec'd out and you dropped the size to 12", I'd be concerned that it is too small and would put in two pipes both 12" dia.

https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/contact/local/