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 Products > Settings > Attributes. The size unit option is available in the Standard Attributes section below the attributes table. Choose m, cm, mm, in, yd or ft (default: cm):
You can also disable the size feature completely. In order to achieve it go to the Products > Settings > Attributes and switch the Size Unit option to Disable Size.
A filled size is shown on the product page. Use the [product_size] shortcode to display it anywhere, and the Catalog Size Filter to let visitors filter products by size. See also Product Weight and Product Attributes.