"Linear inches" refers to the total length of an object measured in inches, regardless of its shape or dimensions. It helps to understand the overall size or length of something in a straight line. Full definition
If you create in a lot of different sizes, you may find linear inch pricing more sensible than square inch pricing. (theabundantartist.com)
In other words, you charge a set dollar amount per square inch (that's the length times the width of your piece), or a set dollar amount per linear inch (that's the length plus the width of your piece). (theabundantartist.com)
For most airlines, carry - on bags have to be less than 45 linear inches in size, meaning that the total length of the bag's height, width, and depth must not exceed 45 inches. (travelinsurancereview.net)