if you have QT Pro and open a movie in it, 
then select movie properties (apple+j)> "Video Track" > "Visual Settings" > "transformation"

it is possible to change the size and go full screen with it.

do you think it would be possible to make a script that would let you choose some pre-defined values? 

I'm afraid this will not be possible as Front Row is very likely not 
using QuickTime Player (= application) but uses QuickTime (= system 
API) directly. It may be possible to write a Haxie using Application 
Enhancer (APE), but this is something I certainly want to stay away 
from.

Best regards, 
Felix