RainRez works in conjunction with the SkinInstaller application which comes with Rainmeter.
This program can be run at all times in the background, and it will monitor your screen resolution and automatically load the "Theme" you define for any given resolution. RainRez is very small and uses very little memory and almost no CPU.
So you can create a Rainmeter "Theme" that fits well when your resolution is 1024x768, with all your skins positioned to fit that screen resolution.
You can then create a theme which fits better if you change your screen resolution to 1280x1024 or some other size.
RainRez will detect when your resolution changes, and restart Rainmeter with the appropiate theme you define.
First, make sure the value for RainmeterPath under [RainRez] in RainRez.cfg is set to the correct path for where your Rainmeter is installed. This will normally be C:\Program Files\Rainmeter.
Then simply change the RainRez.cfg file to reflect the screen resolutions and theme names you wish for each resolution under [RainRez].
One really good option is to use RainRez to start Rainmeter when you start up your computer, by replacing the shortcut to Rainmeter.exe in your "Startup" folder with a shortcut to RainRez.exe. When run after your computer starts RainRez will automatically start Rainmeter with the "theme" appropriate for the current screen resolution.
This is particularly useful for laptop users, who may restart their computer after plugging in an external monitor or connecting to a projection device.