Create a new value and name it as Shell Icon Size. Give same the value that you wish in terms of pixels.
Alternative steps for changing desktop icon size.
1. Right Click Desktop, Choose Properties/
2. In the Properties Window click on the Appearance Tab at the top.
3. In the Appearance Window click on the button called Advanced
4. In the Advanced Appearance Window choose Item: from the drop down list and choose Icon and you can choose desired Icon size. By default many users will see 32 x 32 pixels but this can be changed to whatever you want.