That is what I started with, 2 solar showers and a shower/pottie tent
I plan to use a regular toilet "de-tuned" for water use, so I figured high at 5 gallons of toilet waste for 2 people and a couple inches/gallons of water; so paper and stuff doesn't just ...ugh... stick and harden, so that's about 50 gallons a week(I know it's high better than low)
after dump I plan to add water (fresh would be best, no proteins from grey cooking, washing etc water to stink up the black tank), and this will coat bottom of black tack to help control "stacking")
and for grey, it depends a lot on what we doing? I figure a gallon per night to cook with (pasta, steamed veggies and shell fish etc)
and dish washing 3 or 5 gallons (pure guess) so 50 gallons plus showers etc.
Now so far 90% of our camping has been at FHU sites with bath houses (that is why bath area is so low on the list) I know a lot of folks just pour out strained cooking water so...no discussion on that
So it will depends on how you dispose of water, dry camping with a crate under your shower tent is fine in a lot of places...others it is frowned on
So figure out what you think you will need and go a little bigger