Not sure if a script like this exists.
It may be useful in some cases.
Fits all layers (or 'only visible' layers) to image size.
Install script in your scripts directory
(you can see the scripts directory by going to
Edit -> Preferences
and then once the preferences windows opens,
go to Folders -> Scripts)
How to test:
* Create a new image (say: 800x600px)
* Create another, but smaller image (say 100x100px)
* Copy the small image (Ctrl+C)
* Paste in the bigger image (and create a new layer)
* Duplicate this new layer a couple of times