320*240 or 640*480 Automatic fitting

I have shot clips with my Canon Camera in either of the two above formats.

With QuickTime the format is automatically recognized and dispayed as such.
Pity I have to load one file after the other.

With MP10 all files load automatically one after the other when one
addresses the CD, but the image is maximized and the only way I can get the
display to the right proportions/definition is to shrink the window until it
stops/blocks the display at 320/640 as appropriate. BUT I have to do this for
each file! losing thus the benefit of automatic loading.

How to get MP10 to display to the right size display automatically? Thanks,
Pierre