Start with roll width
Measure the paper across the face of the roll, edge to edge. State the unit and whether the number is a required maximum for your printer. A listed size such as 80×80 usually describes width and outer diameter, but always confirm the supplier’s convention.
Check diameter and core
Measure the outside diameter of a complete roll and the inside diameter of its core, if present. These affect fit in the printer and on holders. If the roll is coreless, say so explicitly.
Complete the purchasing brief
Add printer model, paper grade if known, printing requirements, packaging format, estimated quantity and delivery destination. A sample or photo is useful when the existing roll has no clear specification.