We don’t have a lot of room and I’m wondering what sort of size makes sense. It all comes down to comfort when sitting inside.
It is hard to tell from the photos how people sit in the tubs. If we are to sit with legs straight out then it would have to be quite long. But if it is one we sit upright in, then I would want to make sure that the inner seat is high enough that we aren’t squatting. Does that make sense? Where would I begin to figure this out?
I think that size is a very subjective matter.
We would recommend building a simple mockup with some cardboard boxes, or maybe just with some furniture and a couple of walls, and simulate the size of the ofuro.
Consider that while you are in the water, the body loses its weight (especially in a deeper soaking tub where a larger percentage of the body receives the hydro-static lift)
therefore I would not be too concerned about the sitting position with the legs being straight or bent.
I think what is more important is if the tub is going to be used by one person or there are chances of 2 or more people taking a bath together.
About the internal bench: we usually recommend a simple bath without a bench. It is easier to maintain and to clean and you have more flexibility of use (use in 1 person or 2 etc.)
When you are sitting on an internal bench, the full immersion bath becomes a half body bath (unless you increase the depth of the ofuro). But a deeper tub uses more water and it becomes more difficult to enter/egress so we would not recommend it.
If you like to alternate periods of full immersion bath with periods of half body bath the I think the internal bench is perfect because you can just switch sides.
If you have a preferential direction (facing a window etc.) we can install a removable bench. (but as the wood tends to float it may be cumbersome to insert/remove the bench while the tub is filled with water)