[Thomas Mcdonald]

Sorry this may be a little late… but you can do it by duplicating the mpeg1 preset in compressor, closing compressor, find that preset via MacintoshHD > Users > (your user name) > Application Support > Compressor and open it with text edit and change the parameters according to what you want. (I make 640×480 with 3.5Mbps adjustments to it.) Save that file and when you open Compressor again your preset will be updated. This wasn’t my idea i found the answer on apple’s support forum:

https://discussions.apple.com/thread.jspa?messageID=9909866&#9909866

hope this isn’t too late!