Use size_x/size_y if you want your panel bmap to be tiled over the screen or part of the screen. This keeps buttons at the same size. This is usefull for example for a fullscreen inventory screen or other fullscreen menus. Or for some 2d games that use mostly panels.

Use scale_x/scale_y if you want to resize your panel. It scales the bmap, size and buttons etc. This is for example usefull for loading screens or other panels with only a bmap/image.