The length class is used for your product size details. This Information page can be found under 

 Length class will be available on ---> System ---> Localisation ---> Length class

The length class can be deleted, and new length class information can be added in its place. The length class information should be created before products are added. You can edit the length class by clicking the edit pencil button.

you can choose to  "delete" the Length Class on the top left corner and click the + button to add new.

 To insert a new length class  click + Button, the following screen will be displayed   

  1. Enter the length title name
  2. Enter the length unit name
  3. Enter the value - this option is calculating the default value so set up one default value (Ex: 1cm = 10 mm  this one default)
  4.  Click the save button

Then how to use this length class 
catalog --> product --> Data tab --> give the length class

The result of the customer Length class update can be seen in the screenshot below

In Website View,

In Mobile View,

Congrats!!! Now your product length class is enabled to the public, Which will help your customers make an informed decision with their purchase.