From the main menu click on Units and choose Unit type

Here you could able to set your unit sizes such as Large, Medium & small. Also you can create as your wish

Click on New , then new screen will resemble

Select unit type, then enter the type of unit that you wish to create.

Then, click on save

Then you can view unit type grid with edit and delete option

By clicking on edit, then pop up will open for editing.

Then, you will able to edit the existing unit type and click on update

If you wish to delete simply click on delete and clik on yes in the confirmation window .