How to change the default desktop icon size?

Right click on the icon, and then you will see an option to resize the icon. At least, in 12.04 it works.

For changing the default icon size: In Nautilus (eg your personal folder) > Edit > Preference > tab Views > under Icon View Default change the Default Zoom level.

But this change the size of all the icons inside Nautilus.

But in Nautilus you can change the icon size in a particular folder by View > Zoom in, to get the normal size of the icon in that folder, and this will be remembered after a restart.

-- Open file explorer
-- Edit > Preperences > Icon View Defaults

Set as you wish.