Scale a panel to the screen

I've made a loading screen for my project in the scale of 1024*768 but if I start the game with a resolution of 1280*1024 then the Panel of the Lite-C Logo and the loading screen is too small. My Question: How can I scale the panel to the resolution of the screen?

panel.scale_x = screen_size.x;
panel.scale_y = screen_size.y;

doesn't work really
before it was like that:

but now it's like that:

could it be, that it's because my normal resolution of my desktop is 1920*1080 but the game runs with 1042*768?

panel.size_x = screen_size.x;
panel.size_y = screen_size.y;

The only thing I could think of now.

mhh, no.
on 1024*768 it's normal but if i go higher it looks like this:

menu_panel.scale_x = ((screen_size.x+1)/1280.000);
menu_panel.scale_y = ((screen_size.y+1)/1240.000);

now the panel istn't shown!
Edit: It's shown! I usesd the wrong (size intead of scale) now its looks like it:

oops ssry

menu_panel.scale_x = ((screen_size.x+1)/1024.00);
menu_panel.scale_y = ((screen_size.y+1)/768.00);

good, thanks now it's in fullscreen