How can an arrow be used to represent the size and direction of a force?

So, you can use an arrow to show the magnitude and direction of the force. The direction in which the arrow is pointing is the direction in which the force is applied. The length of the arrow can be used to describe the magnitude of the force. 

As shown in the attached picture, you should a scale to help the viewers determine the magnitude of the force....the direction is self explanatory.