[morzh]

If your resistor does not get superhot, 5W is enough.

here's about the turn ratio: I will try to make it simple. But you have to read it all.

You need to understand what a transformer (AC power or audio output type) is:
- The ideal transformer is an IMPEDANCE MATCHING DEVICE. That is, it takes an impedance, whatever type, and converts is into the same exact type of impedance, but of a different value. At its simplest, it will convert a resistance of X Ohms into a resistance of N-square * X Ohms.
Where N is the turn ratio.

Example: a transformer with the primary of 100 turns and the secondary of 1 turn (100 turns ratio) will convert the resistor of 1 Ohm connected to the secondary into 10,000 ohm at the secondary.
That is, we have just matched a 10 kOhm output impedance, as a Pushpull of 2x42 tubes would like to see, to a 1 Ohm load, which is a typical old speaker (Philco) resistance.
(Again, I simplified the impedance of the speaker, but this is just not to confuse you much).

Why is it? Transformer is the power transfer device. Which means, with the ideal transformer, the power you put into the primary, will be the same, as dispensed to the load from the secondary.
Example: same as above.
You do know by now (it is a common knowledge), that the voltage at the secondary is equal to the Voltage at the primary divided by the turn ratio.

So we have The ratio N = 100.
This means, that a 100V, put across the primary, will result in 1V output from the secondary.
Now, 1V into a 1 Ohm load will create 1W of power. Because 1V/1 Ohm=1A. this is at the secondary and the load.
But at the primary, given the power constant of 1W, you will have 100V and 10mA (0.01A). 100V * 0.01A = 1W.
Now let's look at the resistance: we have 100V/0.01A = 10,000 Ohms.
This is how we converted 1 Ohm into 10,000 Ohm. That is, the conversion factor is 10,000, which is the square of the 100, which is the Turn Ratio.

So, in order to match an impedance Z1 to an impedance Z2, we have to make a transformer with the turn ratio of (SQRT os Square Root)

N = SQRT (Z1/Z2).