As far as I know, the best thing you can do is to have the preview screen active/live when changing channels and then have your settings to 30 seconds and the picture will go full screen. It would be nice if it were shorter, but 30 seconds is the lowest option.
Appearance:
TVguide:
a. Preview - (This affects both the full guide and side guide)
1. On - Gives preview window
2. Animated transition (If on)
3. Autoplay Channels - If on plays channel highlighted without selecting.
4. Stay on TV Guide - If on requires second click to bring channel full screen.