First, you want to set yourself up a set of .reg files for the following icon sizes; 48x48, 64x64, 72x72, 96x96, 128x128
To do this, copy and paste the following into a .txt file:
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Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics]
"Shell Icon Size"="96"
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(Don't copy the dotted lines!)
After pasting that into a .txt file, save the file, then rename it to "Icon96x96.reg". Copy this file 4 times and rename it for each other icon size (Icon48x48.reg, Icon64x64.reg, etc) Open each .reg file you made with notepad and change the line "Shell Icon Size"="96" to a number that matches the file name.
Icon48x48.reg will look like this:
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Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics]
"Shell Icon Size"="48"
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(Don't copy the dotted lines!)
I'm using standard WinXP icon sizes here, you can create a .reg for any icon size from 16x16 to 512x512. I wouldn't go larger than 128x128, but some people do.
Now to change your icon sizes, double click the .reg file for the icon size you want. Let the information be added to your registry. Log off, log in. All of your icons will now be the size you selected.