THanks for the reply, Joe,
The portal.css and default.css files are empty in my site.
When I inspect the table cell using Firebug, the styles are the same both when I inspect the table outside the editor, and when editing in FckEditor. However the computed font sizes from Firebug are given as 10px when outside the editor and 16px when editing. The only css effecting the font-size is from the body {font-size: 62.5%}.
So it seems that FckEditor is ignoring the font-size from the body tag when displaying the table.
Ed