A Cock that is as wide as it is long...
Linz: Oh my, riding you is like riding a cube
John: umm thank you, its beacuse i have a box cock
Male genitalia that is as long as it is wide and can be small or large.
Dude: Baby, I hope you're not a virgin 'cos this bad boy's gonna hurt.
Girl: Oh it's wide... do you have a box cock then?