Unfortunately my AWG chart only goes up to 40GA wire. If you were to use that you would have to use 157 feet of wire per pole to get a 100 ohm armature! I expect that it would not fit, so one would have to use finer wire. If you were not aware of it, when winding armatures the 2/3rds rule is in effect for three pole armatures. When reading the ohms you are actually measuring one pole with the other two in series and connected in parallel. In this case for the arm to measure 100 ohms each pole would actually be 150 ohms.
Modern slot car armatures all have three poles. The armatures have three coils of wire connected in a delta configuration. If you were to isolate one pole of a 16 ohm armature you would read 24 ohms. When you measure the armature you are actually reading all three poles. Two of the poles would be in series, so they would add up to 48 ohms and they would then be in parallel with the third pole. For resistors in parallel the effective resistance is the inverse of the sum of the reciprocals thus: 1/24 + 1/48 = 0.06249999, the inverse of which is 16. When the resistors (actually inductors in the case of an armature) are of equal value the armature must always measure at 2/3rds the value of one pole because of the delta ( Δ ) conection.
I don't think that it would be practical to have a slow car by winding a high ohm armature. With bigger cars you have enough room to have another set of reduction gears, so that the motor could run at normal speeds. You didn't say why you wanted a slow car, but you could also try backing off on the motor brush tension. In the case of inline motors the resulting arcing would damage the commutatir, but with a pancake car you probably would get away with it. You could also try putting a large fixed resistance between one side of the controller and the track. You would have to use a wound resistor.
Since the resistor might tend to get hot you wouldn't want to use a regular carbon resistor. A wound resistor has wire wound around a ceramic core. A 100 ohm one might do what you want. You would have to use something rated at 50 watts or more.