PCB123 currently supports only a single continuous Board Outline. You can modify the default square or rectangular shape to meet your needs as long as the shape is a continuous polygon.
To modify the Board Outline just enable the Select Object tool button and left-click an existing corner to move it to a new location. If you left-click in the center of a segment a new corner will be added to the polygon to allow the creation of a complex shape. Press the "G" key to set the working grid to a smaller value if you need more precision in the placement of the corners of the Board Outline. Any notches in the board outline must be at least .1 wide to be routed through the PCB123 service.
If you would like to create a rounded side or corner right click on the side or corner and select convert to arc.
After modifying the outline you can refresh the mechanical layer by selecting Design Tools Generate Mechanical Drawing. This will update the board size shown in the upper right hand corner of your board outline.
If you have already started a board and want to switch to a round board you can still turn you square board outline into a circle. Start by creating a square outline with the sides equal to the diameter of the circular outline you want. Next, right click on a corner of the outline and from the context menu select convert corner to arc A. You will then be asked to enter a radius, enter the appropriate radius. Then repeat the steps above for each remaining corner. The result will be a round board.
Once you select a corner of the board outline you can move that corner to a specific coordinate on the screen. Press the x key to see the Zoom to coordinate window. Enter in the desired coordinate and press OK. This moves the cursor and outline corner to the specified location. Press the Enter key to drop the corner. Remember that PCB123 always drops objects on grid so you need to set your grid to an accuracy that accommodates the coordinate. I usually switch the grid down to .001".