Robusto extra means a robusto that is longer, and sometimes slightly thicker. There is no single standard. Retail size charts often list it at about 5¼ to 5½ inches long with a 50–54 ring gauge; the Cuban Trinidad Robustos Extra is 155 mm (about 6⅛ inches) by 50. At the longer end it overlaps the toro, and the manufacturer decides which name goes on the box.
The extra length makes the cigar last longer and gives the smoke more distance to cool than in a standard robusto with the same blend. It is a parejo unless the manufacturer says otherwise.
Compare the petite robusto and the robusto on the shorter side and the toro on the longer side. Full guide: robusto guide. Size chart: ring gauge and length cheat-sheet.