Hey Josh,.I think many have used 100 gallon,..but it comes down mostly to your water supply, but also the kind of work you're doing.
Like for me doing house washes gives more time off the trigger than say doing large areas of concrete and running a surface cleaner at full flow. So I'd say, it all depends on the variables.
*House washing with a soap injector slows water usage a small degree,..and also when you take into account brushing gutters, windows etc,..gives a bit more recovery time on the water supply.
**I use 300 gallon, but that is due to having to haul water to several jobs every season. I need it more for the actual supply than just the buffering.
Jeff