given that a 7805 is good for upto tha 1A and assuming you want to use
all of that, 2W would be plenty.
you may want to use a higher resistance (and larger wattage)
so that the resistor burtns more of the excess voltage leaving less
to heat the 7805.
If that battery isn't going to be connected to a generator (like in an
automobile) the transzorb may not be needed, (it won't do any harm
leaving it in though).