You can feed your frog nightcrawlers the same size as the frog, just make sure if your frog is still a baby and they are pretty wide, to still cut them up smaller. Most people feed for around 15 minutes, most frogs will offer clear signals when full, wich those are depends on your frog. Just remember, pacman frogs don't over eat!
Most of mine turn away when they are full, I also have one that squeaks at me.