In Explorer, You can select Large Icons, Small Icons, List, and Details from the View menu. You can alter the size that explorer uses by adding a Value to the registry and/or modifying an existing Value. Both Values are type REG_SZ. Edit:
Double click on Shell Icon Size, if you have Large Icons selected, and change the default (32) to 48 or 64.
I prefer the to leave Shell Icon Size at 32 and add Shell Small Icon Size which defaults to 16 and set it to 32, 48, or 64. This also effects the Start Menu and Open Dialogue boxes.
Both these Values work in consort so you may have to play around if you want to change both.
Setting View to Details and Shell Small Icon Size to 32 works well at high monitor resolutions.
You can also change the color depth by double clicking on Shell Icon BPP. The values are:
If you had Large Icons set and you change Shell Icon Size, the change will happen instantly. For all others, Logoff/Logon.