We need four parameters to describe a paper roll size: Width, Diameter, Length, and Core. And the paper roll sizes can be described in:
Width x Diameter x Core
Width x Length x Core
Some examples are:
For the paper roll size 80mm x 70mm x 13mm, the 80mm stands for paper width, the 70mm stands for paper roll diameter, and the 13mm stands for the core size.
For the poplar paper roll size 3 1/8" x 230′ x 7/16″ in American countries, the paper roll width is 3 1/8", the paper length is 230', and the core size is 7/16".
You can read this article to learn more explanation about paper roll sizes. And if you want to know the mathematical relationship between a paper roll's diameter and length, you can check this link.