I am not sure what you are asking. Image size is in pixels, print size is in inches.
I am assuming you want the print size.
In the Print editor, you can  invoke annotated text and type in 12" x 16"
Also in the Print Editor, you can use Floating Text with the presets for Filename, and a ton of other information, and type in the same box 12 x 16
The advantage using floating text is to be able to place it anywhere, set the font and point size, and the colors.
The other advantage is the presets (for example) Filename, stays ..... and Qimage will put the correct filename on the print regardless of which filename print after print.
Fred

Yes, very much the print size.... In mm or inch depending on what I happen ​to be working in.

My point is that I might print 20 images all different sizes. what would be great is that along with the file name the 'print' size was there as well. This would save me so much time not having to get a rule out measure the image and write it down on a piece of paper.... 20 times!

There wouldn't be much point in typing in the sizes as text, that will take too much time... It needs to be automatic!
It's purely for my information and not part of the image.