...what wattage do I need to have for the resistor, with a maximum of 10W output ...
Easily solved using Ohm's law.
At 10 watts into a 50 ohm antenna, the voltage across it would be sqrt( 10 * 50 ) = 22.3 Vrms.
With 22.3V across a 10K resistor it would dissipate 22.3 * 22.3 / 10K = 0.05W. A 1/10W resistor
should be adequate, but a quarter or half watt would give you plenty of margin. (And that assumes
100% transmit duty cycle - in practice the actual dissipation averaged over a QSO would be perhaps
1/4 of that.)