If you know an actual size and need to find a scale to fit the artwork on a page, use the Measure scale tool to create a custom scale.

Measure scale
This tool works much like a legend on a map. To try this tool, create an object and assume you only know its actual size. Select the Measure Scale tool and click the first point from which to measure. Drag to define the scaled distance. In the custom scale dialog box, the absolute scaled distance will appear. Enter the actual distance on the right side of the equation. This scale will appear as a custom scale in the CADdocument scale menu.