Assuming you use the resistor values suggested by GoldPoint for your attenuator, the dB step size will be the same whether it's a 100K or a 250K pot. The reason it will be the same is that a pot attenuates using a voltage divider circuit, and its the ratio of the resistances that sets the attenuation in dB, not the absolute value of the resistances.
You could select a custom set of resistance values that would change the pot taper if you wanted, making the attenuation steps bigger than Goldpoint's suggested values. Bigger steps means the pot attenuation range would be bigger, that is more attenuation to start with, and so you would have to turn the volume pot higher to reach the point that's now too loud. This will work regardless of whether it's a 100K or a 250K pot. You will have bigger volume changes per step though.
Increasing the pot total resistance would change the input impedance and that could make your system more sensitive to noise, so I recommend against that.
