Sorry I'm not familiar with the Tabbed Docks as I'm only skinning the OD freeware version
but quoting Cavan1 this is the only way:
There is no setting for icon spacing. You would need to edit all your icons onto bigger backgrounds, for a 128x128 icon make a 148x148 trans background for it as an example. This will make icons when applied at current size setting a little smaller but look more spaced out due to the trans area now around them.
