The crosshair on your screen is a GUI object. Its name is "Crosshair".
To accomplish what you're trying to do you'll need to resize the object. You can do this by calling resize(x, y, w, h) on it.
Example: Crosshair.resize( getWord(Crosshair.position, 0), getWord(Crosshair.position, 1), 128, 128);
The first two arguments are the position of the object, all I did was make it stay where it already was but keep in mind if you're resizing the object it's not going to be centered how you want it anymore. You'll have to offset it. You're also going to have to figure out when to run this code, look into when the crosshair is added (hint: onAdd).