When I select a frame size, such as 640 x 480 in *any* video editing program I have ever worked with during the past several years, it *always* resizes the video output to the size I specify.
All Celestia does is place a smaller-than-display-size frame on the current display, which results in only a partial video capture of the display. This is not correct behavior for video frame sizes. It should resize the Celestia display to the size frame being requested so that the entire output display is enclosed in the video frame.