The robusto is the workhorse size of the last few decades: short enough to smoke in a sitting, thick enough to carry a full blend. Common factory numbers cluster around 5 inches by 50 ring gauge, but the name is traditional, not a caliper law.
Because the ring is generous, blenders have room for ligero in the core. Because the length is modest, the cigar does not demand an hour. That combination is why the size took over many lines that used to lead with a corona or lonsdale.
It is a parejo unless a house says otherwise. A “robusto perfecto” is a marketing hybrid; trust the shape you can see.