Circles are placed by pressing and dragging the shape you wish, to create the circle you simply release the mouse button. Circles are created as an ellipse, but you can choose to lock it to a perfect circle by pressing and hold shift while dragging.
Lock aspect ratio
Circles are created with aspect ratio lock on as default, this means that any re-sizing you make will proportionally resize the entire circle. To remove this toggle ‘Lock aspect ratio’ under Edit in properties.