I feed my newts every two days or there abouts, if they are breeding I will give them extra. One large nightcrawler sounds about right for one meal, for a wild newt, a big juicy earthworm would be a rare feast.
I keep my nightcrawlers in the bottom of the fridge in moist (not wet) dirt from my garden. They seem to last indefinitely there as long as they don't dry out, so just mist them once a week or so.