When I work in an existing document I often open a new document and want to paste some result there. In 90% of the time I want to use the Clipboard size to generate a new document and not something predefined like A4, .... For me it would be helpful in both, designer and photo, to have a button to get the size from the clipboard. Now I have to look for the sizes before, not them down, create a new document, write these values there, create it and past it.
A similar workflow you can see in Paint.NET (win only). When you copy something and create a new document it automatic set the size. I recoment to not automate it always, just a button to set the fields with a clipboard size.