Handling screen resolution through a script

hi all
is there a script which would allow me to handle different screen resolution like 1024x768, 800x600 to a fullscreen?
my probelm is: i have a client for whom i am doing the cd presentation.They want to use full screen for presentation i.e is the canvas size should be more than 800x600. but my issue is if i am using more than 800x600, wont its side get cut in the same resolution itself?
so i want a script wherein i use fix canvas size of 1000x750. n script handles the 800x600 resolution too…i hope all this is not confusing…but pls pls pls help me with some solution…its ■■■■ urgent…