Heater voltage.

[Mike Watterson]

50W at 250V = 5000/25 mA = 200 mA
A 30V drop at 200 mA = 150 Ohms
That's 6W.

There are 5W, 7W and 10W wirewound resistors. Two 68 Ohm 5W in series or two 5W 270 Ohm in parallel is fine. No need for a heatsink,