Take a look at the
Sarkes Tarzian Selenium rectifier databook - most seleniums have about a 5V drop across them, and a bit lower when leaky.
Assuming a 5V drop, your 320ma (from an earlier post), and you have your two variables needed to get started. A silicon diode (1n4004/5/6/7 series) drops approximately .7V, so you need to drop the other 4.3V.
For your series resistor, R=E/I, so 4.3, divided by your current 320mA = 13.5 ohms, roughly. Power is as easy as P=I X E, so 320mA X 4.3V = 1.5 Watts, better to upsize it to a standard wattage. I'd go with a wirewound, as most filament resistors of yore are wirewound:
http://www.talonix.com/images/RESPFIL.jpg
I'm surprised there isn't more info (reliable, that is) about replacing seleniums on the web. We used to replace them all the time when I worked for my dad - even attached a new schematic for the next tech, so he'd know what he was dealing with. My least favorite swap was in a Nutone intercom for a dentist - the selenium was buried under a multi-station switch, with little room, and harly any room to mount the resistor. We went with a chassis mount, and used a resistor only a watt above the calculation. Solid state intercom, but it had a selenium rectifier - go figure!
Cheers,