Fairly newbie question here! I always use a small number of layers when painting, never more than 3 or 4.
However I'm wondering how to select something across all of these at once, so that I am able to resize selections on the whole image.
You are able to resize selections by using the "Selection Adjuster" tool (the shortcut key is "S" by default, I believe); however this will only change the selection size and shape, not the content within.
Unfortunately, to my knowledge you are not able to transform more than one layer at a time in Painter 12...this is only possible in Painter X3 or newer.