Switching to the Grid View in iTunes 9 or 10, you’d likely think that there isn’t a way to change the size of the individual album art and make them smaller or larger, but there is. To be able to do so though, you’d first have to select View > Grid View > Show Header [...]
From a 20-in. aluminum iMac to a 2.4 GHz, 15-in. MacBook Pro (2008) and 2.93 GHz iMac (2009)