I am doing this in Windows 10 but I assume it will be the same as in Linux. The program assumes a resolution of 640x480 which is quite small for most monitors. A quick fix is to put this code in the program:
$RESIZE:STRETCH
With $RESIZE:STRETCH the screen will be stretched to fit the new window size with a 1 to 1 ratio of width and height.
A better option would allow you to have a program option to select the window size. For example, 1280x960.