If none of the wizard templates matches your floor plan exactly you may choose the rectangular template or another one being similar to the actual room shape and adapt this to the actual shape. For this activate the menu item Floor Plan in the menu Draw.

Select a corner point with the mouse to move this or click on the inside edge of a wall to insert further corner points.
Using 'Guidelines' facilitates to meet exactly the correct measures of your floor plan.

You can insert or remove 'Guidelines' with the left mouse button if you click on the rulers. In the menu View you will find some further tools which provides for additional help for the room design.