If you are comfortable editing the registry you can find two values here: HKEY_CURRENT_USER\Control Panel\Desktop\Windowmetrics
Shell Icon Size defines the size of normal icons.
Shell Small Icon Size defines the size of small icons such as when you are viewing listview.
Source: http://www.virtualplastic.net/html/desk_reg.html#metrics
You will have to reboot to see the changes.
If you don't want to edit the registry your self you can grab yourself a copy of FreshUI. http://www.freshdevices.com/freshui.html
You can edit the icon size from Windows Interface - Desktop - Icons - Icon Size
It will ask you to register and expire after 30 (or 60) days or something, but if all you want to do is change that setting you won't have to worry about that. It's free to register the app anyways.