Run command if potion effect is over

I’m pretty new to modding etc but I’m using pehkui to make potions that scale you up and down when drank. My issue is that when the effect runs out I want to reset their scales back to normal (the pehkui plugin has a block for that) but I just don’t know how to get it to trigger? Im probably missing a really easy fix but if anyone knows how to help you'd be much appreciated!