Apologies, by dimensions, I did mean aspect ratio. All I mean is keep the same image, but reduce the number of pixels, and lose some clarity, to reduce the size of the file.

Your suggestion above doesn't work. Try it. It keeps the picture aspect ratio, but it crops the picture. I don't want to crop it, I want to shrink the number of pixels but keep the same image (with loss of clarity because reducing pixels).

Mhh. Maybe if you can show me an editor/app that does what you require will help me figure out how it works and replicate it

MS Paint, Resize/Skew option. Ctrl-W is the hotkey.

It allows you to resize either in terms of pixel count or percentage. The percentage approach is very nice for just putting 50 in both horizontal and vertical fields and getting an image half as large in both dimensions, but showing the same image (with reduced quality, because fewer pixels) and at the same aspect ratio.

done! added in the "composition" menu and fully integrated in the workflow

I preferred just having an input with the correct pixels rather than % (to keep UI clean) but you'll get the same result