PROJ TYREPEAT

PROJ_TYREPEAT is a defineprojectile property used to set the yrepeat of any trail sprites spawned.

defineprojectile <tile number> PROJ_TYREPEAT <number>

The y size of the trail used by the projectile. Note that if the trail is an actor with a size specified in its code (e.g. by a sizeat command) then the specified size will override the one you give it.