I want to reduce the canvas size (and the background layer) from 500×500px to 16×16px. How to resize the canvas in GIMP?
If you want to resize the contents as well (layers, paths…) see
Image>Scale image. If you want to just reduce the canvas size, see
Image>Canvas size. If you have a 16×16 layer in the middle of large canvas,
Image>Fit canvas to layers can also be a solution.