Outlined / unfilled circle in photoshop

Take ellipse tool and drag it with holding shift key for any size you 
 want, anywhere in your document. 
 
Set its fill property to "0".
 
This creates the empty circle.
 
Now go to menus Layer --> Layer Style --> Stroke
 
A property window will open.
 
In that window, set circle's line properties as per your requirement.  In particular,
adjust the width of the line and the color.
 
 
As a side note, if you want to fill it then set its fill property to 100,
then write over the circle.