By "player screen", do you mean the desktop? These Windows commands get you the current desktop width and height:
int DispW = GetSystemMetrics(SM_CXSCREEN);
int DispH = GetSystemMetrics(SM_CYSCREEN);
In the newer versions of Dark GDK, these should work as well:
int DispW = dbDesktopWidth();
int DispH = dbDesktopHeight();
Then you can set your Dark GDK window to the same resolution (for example with dbSetDisplayMode or dbSetDisplayModeAntialias).
However, this won't solve a sprite sizing problem. You design your game for a certain number of pixels, a certain horizontal and vertical resolution, on which it looks right. If the program is run at a different resolution, sprites will be either too big, too small, or distorted (if the aspect ratio is different).
One solution is to run your game in a non-resizable window, i.e. to force the player to use the same resolution as you. The other is to use the desktop resolution of the player, but make black bars around the image if it's smaller than the desktop.
