Changing template spine size.

[Stusamuso]

After reviewing one of my indi books, I want to increase the spine width from .835"/21.21mm to 0.880"/22.35mm in my Gimp 2.10.36/Windows11. I am reusing my original book template so the only change required is the spine size (more pages in the new edition). How can I best do this in Gimp? Step by step would be good. 
Secondly when I open a template image as a PNG in Gimp, how can I view its actual size?  Thanx, Stusamuso.