This is a matter of opinion and size of home owner, basically I would want a bench that sat like a chair depth in a kitchen, at least the depth of knee to hip

I usually make them 10-12" deep, depending on how much it encroaches into the shower, and about 20-24" high. You also want to slope it very slightly to allow water to run off. If it's in a steam shower, they're often taller/deeper for comfort while sitting, whereas if it's more of just a convenience ledge, shorter and less deep may be better.

A dinner plate is 10-12 inches, lol..does your rear fit comfortably on that??? Yes I have seen them that size,but if I'm choosing I would want one a little bit deeper for comfort and stability

As we age our needs change too