If you are using Windows 7, this process is much more simpler.
All you have to do is right click in an empty space of the desktop. A pop up window appears. Go to View. Three options are available like Large icons, Medium icons, Smaller Icons. Choose what size you wanted.
This following image explains you.
In case, if you wanted to change the size of the icons inside a folder window in Windows 7, do as shown below.
In the right hand side top corner of the folder window,there is a option "Change your view."
If you press the drop down arrow i.e. "More Options."
A list pops up.
You can notice that the list has a size increasing and decreasing bar using which you can increase or decrease the size of the folder icons.
Chandru Always Rocks