I found it best to Always Record in Full Screen with what monitor you have and then resize in the canvas area to what you want OR zoom in to the size you want if that does not cut off too much of your video. This gives me the cleanest video.
"Images and Videos on the TechSmith Camtasia canvas can be easily resized using the handles on the perimeter of the media. If you wish to resize the media independent of aspect ratio, hold the Shift key, then adjust the size using the perimeter handles." ( TechSmith )