The cable can be up to 6.5mm in diameter. The nail should be at least 20 mm long to properly hold the clip in place. The hole is designed to fit at most a 2 mm thick nail, depending on how much the used material will expand.

Personally, I used a 1.8mm thick nail and it fit right and tight. Used 3DJake ecoPLA as filament, printed on a Prusa Mini with 0.15 / 0.2 layer height.