Sorry but you are "just using it completely wrong". :) There are at least two ways to make a circular board outline: 1) Top tool bar > Tools > Set Board Outline... There you can specify the Radius and Centre X and Centre Y co-ordinates:  Or use the Circle tool. Then specify the Radius and Centre X and Centre Y co-ordinates of each half circle in the right hand panel: 

Hi At present, you can't change the radius size smaller, but you can change it bigger, we will change its behaviour at v6.1

How can I create a complete circle, not two arcs that are combined into a circle?

@bkcarter333 The copper layer can't not draw the complete circle now, must combine with two arcs. we will change it in the future.