I always just develop to the lowest resolution I want to support (1024x768) and keep it static, or use sliding objects that resize as the screen resizes.
If you really want to redirect to different layouts, then your trigger script would look something like:
if [ Get(ScreenWidth) < 1024 ]
Go To Layout [ form - low res ]
Else If [ Get(ScreenWidth) > 1023 and Get(ScreenWidth) < 2200 ]
Go To Layout [ form - med res ]
Go To Layout [ form - full res ]
Make sure to read through the WebDirect guide, it can probably answer a ton of basic questions for you.
How do I Create different layouts in different resolutions?
I have yet to find a good example of using OnLayoutSizeChange for web-direct solutions. And FM excluded any Web Direct solutions in the FM 15 Training series. If anyone knows of a good example of using this method to deliver proportionately sized layouts in webdirect. PLEASE HELP...