Grand Marshal is a hypothetical military rank which is mentioned occasionally in the universe of Star Wars. Confined mainly to the Star Wars Expanded Universe, the rank of Grand Marshal is considered the starfighter equivalent to the Imperial Army and Navy ranks of Grand General and Grand Admiral.
In theory, a Grand Marshal would be the supreme commander of all TIE fighter forces in the Galactic Empire. This assumes that the TIE fighter forces are not part of the Imperial Navy, but rather a separate branch of service with their own ranks and chain of command.
The rank of Grand Marshal has never appeared in any live action productions and only briefly in novels and comic books.
No insignia for the rank has ever been discussed or revealed in Star Wars sources, but it is reasonable to assume that, if such insignia did exist, it would likely be the same as Grand Admiral and Grand General.