One thing that will determine your tanks is where you have room for them. What bus do you have?
I think the short answer is, as large as you can safely fit.
I tried to get tanks from cheaper sources, but I was not able to find ones that would maximize the available space, so in the end, I went to tank-mart.com for black and grey and to ronco plastics for the fresh tank.
I found direct experience from Rolling Vistas on YouTube for fresh water tank size. They have a good video on dry camp water usage.
I’ve been looking at a trigger sprayer for the shower. While looking for other things for a construction job I came across bidet sprayers to use for a hand shower. I think that this is a good solution to conserve water