Pipe sizing can be hard. I cannot speak for professional plumbers, who can probably figure out pipe sizes in their sleep. But for home renovators, it's a bit of a challenge. PVC has sizes printed on the side - except when it's smeared off or not there in the first place. Metal pipe is usually a mystery, too. So there is a product called Pi-Piper which measures pipe sizes in an attempt to dispel the mystery.

Image Copyright Pi-Piper; Courtesy Pi-Piper