100 amp is more than big enough unless you have added stuff with a big amperage requirement. Remember that the alternator will only supply the amps that your electrical system requires. So the extra capacity will never be used.
As for supplying amps at low revs, you can change the pulley ratio but I imagine the manufacturer has already chosen the correct ratio to drive it fast enough at idle. Of course it's possible that if it's not designed for the application then it will be wrong.
To work out the amperage of your cars electrical components you divide the wattage by the voltage of your electrical system. So a set of 100 watt driving lights you would divide 200 watts by 13.5volts. So roughly 15 amps. Power windows etc draw a lot but they only run occasionally too.
I upgraded my WB to a 80 amp years ago after the standard alternator died. I had added AC and a set of Cibie driving lights plus put bigger globes in the headlights.
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