What is the best size of the grey water blue bottles?
 

We have used a 10 gallon for many years. Bigger is not always better, and this is one of those instances. I can easily lift the 10 when full and put it on/in the truck. If it is too heavy, just put in what you can handle. Making more than one trip is a minor inconvenience, but that is just part of our dry camping experience. I usually can wheel it to the dump station.