We had a Sketch file with hundreds of Artboards at the same size (our IconFont master document).
When we tried to decrease their size uniformly, Sketch's Scale Tool didn't give us the options and result we wanted: it adds up the sizes of all selected Artboards and the space in between. And using percental values led to uneven Artboard sizes.
So we built this little plugin that allows you to select several Artboards (of the same or different sizes) and scale all Artboards to a single target width and height defined via the plugin dialog.
Enjoy.