Originally Posted by
Chataignier
I have had this discussion with colleagues recently, from my point of view a digital image does not have dimensions in inches or centimetres, it only has pixels and a ratio between the width and the height. A digital image with say 6000x4000px can be printed to paper in 3x2 format at any size : bigger and the resolution is lower, smaller and the resolution is higher. Physical size has no meaning at the pixel level.
Affinity offers 2 re-dimension options, change the number of pixels in the image ie change the resolution, and change the number of pixels in the "canvas" ie add a white space around the image or subtract from (crop) the image. These are the two things you might wish to do and have no bearing on the size of the paper you might eventually print onto.
If you want to crop off part of the image at the same proportions or even to change the proportions of the image, much simpler to simply use the crop/framing tool.