[Radioroslyn]

Well the things to consider are the inductance of the field coil. Higher the inductance the better it will filter. Generally the higher the resistance the higher the inductance. It's current carrying abilities. Wouldn't want to use one from a 20 tube set on your set. The current demands wouldn't be enough to energize the field get full magnetism.

The simple answer is like 750-2000 ohms and from a 5 or 6 tube set.
Terry