It appears to me that the intent is to insert an adjustable resistance in series with the field rather than to power the field from a voltage divider. That being the case, either a rheostat or a potentiometer could be connected to do this.
As far as power rating goes, the Utterpower website says that the field current is 2.1 Amps without the added resistance. With the resistance installed, the current would be reduced--although I can't say how much.
To figure current carrying capacity in rheostat mode you should really look up the manufacturer's specs. However, a rough approximation is to figure what the current would be at full rated power through the full winding. This tells you how much current the winding can stand but does not take into account what the wiper can take. So, in this case a 5 Ohm device rated at 50 Watts comes out to about 3.2 Amps. Lacking more complete information, I would say a 50 W device would be OK (although it is a bit more stressed than I like).
For a real world comparison, an Ohmite RJ series rheostat is rated at 50 W and the the manufacturer gives the 6 Ohm version a 2.88 A current rating.
For what it's worth, when I use a potentiometer as a rheostat I like to connect the wiper to one end of the winding. The advantage is that you don't end up with an open circuit if the winding burns out.
Best regards,
Andy Hall