What voltage drops across it at idle? If you don't know, tack your 10-watter in there for test purposes and find out. That will let you calculate the dissipation. Then at least triple the wattage from what you calculate there.
SOunded familiar, and sure enough 60 ohm 10 watt was used in a Crate VC series with four EL84. When I converted a PV Classic 30 to cathode bias, I raised it to 100 ohms so the tubes ran a bit cooler.
And I would not be averse to using one of those aluminum finned deals from Dale or similar, but then just for size I'd use 25w, they are very compact. Mount to the chassis and run two wires.