Yeah, that's the other thing: GFI's in standard boxes.
It's a good idea to use a larger box but not mandatory. Most of us will learn by experience in this regard because you will often have a ground fault after cramming all of that into the box!
We don't need the NEC to dictate our every move do we? That book keeps getting thicker & thicker as it is...
Anyone who has ever installed a GFI receptacle in a standard sized weather-proof box knows that if you have more than one cable entering the box you will have one helluva time getting that receptacle into the box. And so you add a weather-proof extension ring and you're good to go.
Same with an indoor box: Use a 20.5 cu. in. or even a 22 cu. in. if you have more than one romex to contend with. You'll be glad you do at trim-out.
:~)
