Hello,
GoToMyPC captures the screen as it is rendered by your computer's video card. Although it may vary depending on your model and driver, most video cards will not render a full resolution if the video card does not detect that a monitor is attached. Additionally, even if a video card is capable of rendering multiple monitors it will only render one if multiple monitors are not physically attached.
You may be able to change the resolution and settings manually in the display control panel. If you consistently don't plan on having a physical monitor attached you may want to invest in something like a "dummy plug". These devices "trick" your video card into rendering a display that is not actually attached. You may need multiple plugs in order to have multiple monitors rendered. Different models and manufacturers sell these, here is one model:
http://www.amazon.com/CablesOnline-Graphics-Display-Detection-VE-D01/dp/B00EAY13GC