Can't Changing Frame Size in CCE-SP2

I have a AVI file with 512*384 resolution. I load it into CCE SP2 using these settings:
However if i choose MPEG2 and check mark the For DVD box, I can adjust the first attribute in the Frame Size box, but not the second attribute, unless i uncheck “For DVD”

Is there anyway around this or do i have to uncheck “for DVD”?