Extend the rod:
In most windows, the rod ends at the end of the window frame and the curtains take some part of the window. Extend the window at equal distance from both sides. This way when you pull back the curtains, they will not hinder part of the window itself and will portray a larger window because mostly human eyes judge the size by the curtains. However, do take care not to open the curtains past the frame. It might become obvious to onlookers that you are actually utilizing the wall for creating an illusion.
Buy curtains in the color of the wall next to the window. This will make the window blend in with the wall and it will hard to judge where the window starts and ends. Refrain from using thick curtains with bold colors and patterns because they will make your window look smaller.
Use cafe curtains to your benefit. Place the rods of the curtains outside the window frames so that the curtains are extended. Since cafe curtains come in sections, you can keep the upper part open and the lower part closed. This way nobody will know that the lower part is actually covering a portion of your wall and not the window frame.
Go for horizontal blinds. Horizontal objects suggest width. Blinds also let you enjoy the full size of your window because when you open them, they do not take as much space as curtains do on the sides of the frame. If you do no want blinds, get curtains with horizontal stripes which will show your window much larger by tricking the eye.