The other day the discussion on 40 meters turned to wire size of wire antennas and how it affectes the gain of the antenna.  Thanks to Roy Lewallen and his program, EZNEC, I decided to look into this.  I constructed a dipole in free space and over a regular ground at 100 feet  for each of the bands shown in the graph below, and varied the size of the copper wire used in its construction.  I also recorded the antenna gain for a lossless wire.  The graph below shows the relative loss for several wire sizes and several HF bands.  As you can see, the wire size has very little effect on the antenna gain unless you go to extremes.