Iain,
is there a way to show the file size of each image easily in the edit viewer?
If by the 'edit viewer' you mean the Preview window and its Info text pane, then yes you can. You just need to customize the Info text to include the use of the {filesize} variable.
You can do this by choosing 'Set Info Text...' on the Settings submenu of the Edit menu.
HTH,
-Kirk