The robusto has been the most popular size of the last few decades. It is short enough to smoke in one sitting and thick enough to hold a full blend. Common factory sizes are around 5 inches long with a 50 ring gauge, but the name is traditional, not an exact specification.
Because the ring gauge is wide, blenders have room for ligero in the center. Because the length is moderate, the cigar does not take an hour to smoke. That combination is why the robusto replaced the corona or lonsdale as the leading size in many lines.
It is a parejo unless the manufacturer says otherwise. A “robusto perfecto” is a marketing name for a hybrid shape; go by the shape you can see.
Full guide: robusto guide. Size chart: ring gauge and length cheat-sheet.