as far as i know, to resize panels you should set its scale_x and scale_y

for your purpose this should work
Code:
panel.scale_x = screen_size.x / panel.size_x; 
panel.scale_y = screen_size.y / panel.size_y;


im not entirely sure if the size_x and size_y components change value after the panel is scaled, but if it is, then, if your panel is 64 by 64 pixels, just do it like this instead

Code:
panel.scale_x = screen_size.x / 64; 
panel.scale_y = screen_size.y / 64;

Super Funkybox 2000