actually all you need is a function that sets a panel to resize according to the screensize / the size of the panel like this:

panel* pan;

function resize_panel(pnl, scale_)
{
pan = pnl;
pan.scale_x = screen_size.x / pan.size_x + scale_;
pan.scale_y = screen_size.y / pan.size_y + scale_;
}

you'd use it like this:

resize_panel(panel_name, 0.2);

and if you use it in a while loop, or update it every time the video mode changes, then it will always be the size of the screen. to center a panel is just as easy. you'd use negative values for a smaller panel, or could use 0 for a fullscreen panel...