encoding to full screen/keep aspect ratio with not standard ratio?

[Mr.Big]

I have a .mov that is 640x360, somewhere between 16:9 and 4:3., which I am trying to convert to an MPEG for burning on a VCD I can correctly set the resolution in the source, but i cant customise the aspect ratio so that fullscreen keep aspect ratio will work... i can use center custom and specify the output resolution, but im not sure how big the final video will be compared to the full size of the screen.

thanx for your time,
Mr.Big

[Mr.Big]

actually, even the center custom wouldnt give me the right aspect ratio even if i put the correct res, i had to put both to 1:1, put source as 640x480, and then put output as 640x360, im gonna see what this gives me (finished result) and then ill post here again.