max gauss of a permanent magnet dc motor field?

The limits involve saturation of the iron core of the coils and the risk of demagnetizing the permanent magnets, and of course, temperature rise due to resistance.

The limits are in ampere turns (total amperes circulating around the core), and that depends on the geometry of the motor as well as what it is made of.

I think they do. (excerpt from the following article) "The actual motor design is a radial-flux construction, air or water cooled, with a permanent-magnet rotor. Powers range from 22 to 670 hp (17 to 500 kW) and include base speeds from 220 to 600 rpm with voltages ranging from 380 to 690 Vac."

This is talking about permanent magnet synchronous AC motors, but add commutator and brushes and you have a DC motor.

Here is a permanent magnet DC motor designed for stuff like fork lifts:

Hi John. Looks like the Lem motor is only a couple of HP. Evidently a couple hundred amps is typical in forklifts, so now we know the amps. How about turns? My goal here is to get a number for the strength of the permanent magnet field in these 2 HP motors, so I can extrapolate what it is in the 20 and 40 HP motors. Can you hazard an educated guess?

The field strength is limited by the saturation flux of iron:

The demagnetization resistance of the permanent magnets is also a limit. Neodymium iron boron is especially good in this respect and has really increased the power available from a given size of motor. The resistance to demagnetization is probably best captured by the Hc value:

The maximum field strength for these two reasons applies to any size motor. If they are designed to the limit of the materials, the same maximum flux occurs in all sizes.