Does this apply to all files put on the desktop or just the one icon you're showing? If the latter, that probably means you did it accidentally. Right click the icon, then select "Restore Icon's Original Size...".

The first one, I don't know (yet) how that would happen.

Also, maybe you could try gconf-editor. In it, go to apps>nautilus>icon_view. There's a setting for icon size there. Maybe that's what you need.

If you are using dconf-editor, the path to "icon_view" setting is org>gnome>nautilus>icon_view