Photoshop :: Getting Target Size One Doc To Another

I need to automatically rezize a photoshop document by using an action. Sounds pretty simple, yes. But the thing is that the flow needs to grab the individual/specific size from ANOTHER open document (different from time to time) to be used as the target size.... I need to make the grab the target size from a variable, so to speak.

I've tried to use the crop-tool to grab size and then use select all + crop on the other document, but no. Plus loads of workarounds.

It has to be a full auto function to be run from a droplet.

This is what it will be used for:

A faksimil scan from a newspaper page needs to get a broken side like it had been ripped out by hand. A shadow on the ripped edge as usual. Easy to do by hand. But the action flow will use a "template"document holding a pre-made layermask built with the right edgelook. The Action will rezize this to fit the original faksimil dokument and apply the ripped edge+ shadow on the faksimil document in one go. By loading the selection from the layermask over to the faksimil.