It's not the size of the cup, it's the amount of coffee in it. But if you want a more objective answer, things start to get murky because different coffee experts define a cup different things depending on their agenda.
Most of the big-name percolator companies including KitchenAid, Bunn, Mr. Coffee, etc. advertise a "cup of coffee" as being 5-oz. because it makes their coffee pots seem larger.
Folgers says a "cup of coffee" is 6 oz. because they work with these percolators, but they want people to use more coffee grounds.
Starbucks sells cups of coffee at 8 oz. at the lowest because what do they care? They want to make customers happy.
It gets even more muddled when you start getting into commercial urns and percolators who consider a cup anywhere between the demitasse size of 2 ounces to the imperial size of 8 ounces.
It's actually a very controversial topic. There are some camps who believe that a cup is 5 ounces always and there are other camps who think we should only go by the 8-ounce cup for simplicity.
If you're interested, there's more information about the debate here: http://www.jesrestaurantequipment.com/jesrestaurantequipmentblog/coffee-carafe-sizing/