The actual pixel size of the logon icons are "Etched in Stone" in logonui.exe and cannot be re-sized.
The picture that appears as an icon can be changed.
There are 3rd party programs that can be used to manipulate the log on screen, here are a couple: http://sarahlane.typepad.com/sarah
[...] xp_bo.html http://www.screenbooty.com/
(Thank you Tigsounds)