Script for use with the default Sketch iOS App Icon Template. Takes the selected layer and duplicates it to the other artboard sizes.

Resizer.cocoascript.js

var original = context.selection.firstObject();
var originalArtboard = original.parentArtboard();
var artboards = context.document.artboards();

var enumerator = artboards.objectEnumerator();

var artboard;

while((artboard = enumerator.nextObject())){
	if(artboard.isEqual_(originalArtboard)) continue;

	var originalSize = original.frame().width();
	var targetSize = artboard.frame().width();

	var clone = original.duplicate();

	//	resize

	var ratio = targetSize / originalSize;
	[clone multiplyBy:ratio];

	//	add to artboard

	clone.parentGroup().removeLayer(clone);
	
	artboard.layers().addObject_(clone);
}