How to calculate the required resistor?
I am not sure that there is a quick and easy calculation that can be done without knowing a lot more about the transformer.
If you really want to use that transformer to to give a sinewave, the easiest way would be to experiment with a few different resistors and see which works best.
To start with, try something between 10 and 100 Ohms and see what happens.
If the resistor gets hot and smokes, you need one with a higher power rating.
Of course, putting a resistor in series with the primary of the transformer will reduce the voltage regulation. So if you try to draw any power from the secondary, the secondary voltage will fall. Just like AGs rice cored transformer.
JimB
PS, Anybody know the magnetic permeability of rice?