Once I figured out that the Crop icon on the left of the window also led to the image resize functionality I tried resizing my image.
However after several attempts to 'Apply' the new image size it resolutely refused to change to the new size. Exporting the image it was still at the old size.
I realised that if I scaled to half the number of pixels in any dimension then I got the actual dimensions I wanted when I exported. Has anyone else have the same problem?
Is the scale interacting with the zoom functionality?