Each product can have the size defined: Length, Width and Height. The size fields appear inside the product details box on product add/edit screen:
Set Catalog Size Unit
In order to change the size unit go to the Catalog Settings > Attributes. The size unit option is available below the attributes table:
You can also disable the size feature completely. In order to achieve it go to the Catalog Settings > Attributes and switch the Size Unit option to Disable Size.