1. Go to Start Menu, Run.
2. Then type in regedit
3. Browse to: HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics
then alter the key "Shell Icon Size" to whatever you want, 128 x 128 isn't the limit. It will need a log-out and login or a restart or a restart explorer to take effect.
I must warn you though that it is 128 throughout the system, and that means your start menu, desktop, quick launchbar, there are ways around it though (with comprimises). Such as, setting all folder views to "tiles", disabling autoplay, disabling the "place" bar, getting rid of the icons on the right had side of the start menu.
However i suggest that you just stick to 48x48 or 72x72 like Mavis, depending on what works best on your machine...