Just 4 steps: 

1. Right Click Desktop and Choose “Properties”.
2. In the Properties Window click on the “Appearance” tab at the top.
3. In the Appearance Window click on the “Advanced” button.
4. In the Advanced Appearance Window choose “Item:” from the drop down list and choose “Icon”, then the desired Icon size. By default many users will see 32 x 32 pixels but this can be changed to whatever you want.