5 mm refers to the thickness/diameter of the carabiner body, then you can describe it as:

5 mm Mini Aluminum Carabiner Snap Hook – Black

* Body thickness: 5 mm
* Material: Aluminum alloy
* Gate: Steel spring/wire gate
* Color: Black
* Type: D-shaped accessory carabiner / snap hook
* Use: Keychains, bags, luggage, bottles, lightweight accessories
* Safety: Not for climbing or load-bearing/personal safety use