What are the exact measurements? I think you already know the answer, which is that keeping it maintained in that tight of a space is going to be nearly impossible. Because it's inside the shower, it will constantly be getting wet, which means it will constantly need to be cleaned. If it's not easy to maintain, it's not a good design.

I would say 12 inches would be best to aim for around the tub, some people are okay working with smaller. Just know that the smaller the clearance, the harder it will be to clean.

1 BIG shower. Don't do the tub inside, unless you have a lot of space.

We have an under-mount soaking tub inside of our master shower enclosure. I liked the idea of freestanding, but we didn't have enough room. We used a marble slab as the top, which also provides a bench and area to place bath items. We really like the set up and it is easy to clean with the solid slab top. The attached photo is before it was finished, we now have glass enclosing the entire shower/tub area, with the entry door in the shower area.

Stupidest idea ever Get a tub/ shower combo and save yourself hours of frustration trying to keep all that clean