There are three combinations you get here (Extended Cab, Regular Cab, Short box, Long Box)
Regular Cab Short Box (6 foot box)
Regular Cab Long Box (7 foot box)
Extended Cab Short Box (6 foot box)
Both the Regular Cab Short Box and Extended Cab trucks use the same bed. There is no Extended Cab Long Box.
It's rare, around here anyways, to see a long box Ranger and they've always been Gen I or Gen II. I don't ever recall seeing a Gen 3+ long box Ranger that I remember, although it was an option I'm sure.
The 6 foot box IS the shortbox. It's a reference more for a full size truck where a long box (8 foot) is a lot more common. As I stated above, I see long box (7 foot) Rangers so rarely that if you tell me you need a box for a Ranger, I'll assume you're talking about a short box (6 foot) unless otherwise explicitly stated.
Does that clear everything up?