Change the canvas size
For best results, set the canvas size to the dimensions you plan to export your final video to before editing. Set the canvas zoom scale to 100% for a preview for how the final video will appear produced at the current canvas dimensions.
Change the canvas dimensions
Select Edit > Adjust Canvas.
The Canvas controls appear under the canvas. Camtasia dims the entire application except for the canvas area. Enter the dimensions you plan to export your video (in pixels) into the Width and Height fields or select a common export dimension from the Preset menu.
Click Apply.