How much tonnage air conditioner is required for 2779 sq ft?

It depends a great deal on where you live and how well your home is insulated. Square feet is not a great help when planning the size of an air conditioner. It is very useful to also have the cubic feet in the home, along with the sizes and types of Windows.

Where I live in the northwest US, a 3-ton unit would do the trick easily, but then, we do not get the intensely hot, long summers as you might in say, Phoenix.