Adjusting the Skin's Size to your Screen's Size
This is a very simple adjustment, ant it should fix BOTH issues of when the skin doesn't fit and when the skin is not big enough:
-Open the GENERAL.INI file under your Skin's Folder
-Modify the SHOWWIDTH and SHOWHEIGHT settings to reflect your monitor's Width and Height (Resolution).
-Modify the Day/Night Gamma adjustments as you please (Optional)
When changing the size of display of a skin, it is possible to see distortion in the images (of you stretch it out of proportion). You should look for a skin that matches your monitor's aspect ratio for best results.
On the latest versions of RR, it is also possible to run RR at a different resolution than what you run your desktop applications.. all it takes is adding a line to RR.INI and RR will auto-switch the resolution when running to the specified setting: