If your going to make a video that you will play on your DVD player/TV 640x480 is plenty big enough as Standard definition is only 480 lines anyhow unless you want to play the video on your PC only then going bigger is better.
To lock the screen resolution in TRS use the "trainz Options" text file located in the Auran folder and add the line
-width=640
-height=480
or whatever you want to use (someone correct me if I am wrong please) the Configure Trainz menu should work from the start splash screen. I do not know why you can't get it to work from there.