Open floor space in general needs to be 30" minimum in either direction. A general rule is that a 30" diameter disk should be able to lay flat when placed on the shower floor. Have you considered not building the bench in as a permanent part of the shower and instead having a moveable wood/teak bench instead? You could do a 2-piece bench, even contour it for body comfort. Or a custom-built, wall mounted, fold-up. Flip up when you want to maximize floor space when showering, fold down when steaming.

If it's because of the shower curtain, you can get by with a Kohler pan. The curb is about 4" wide on their Salient pan and you can set the curtain rod back a few inches. We just installed a Kohler Rely acrylic pan. It has a little edge to stop water from escaping. Here's my post from 2016 so you can see I'd been thinking about a shower pan with a shower curtain for a long time before we decided to use glass doors. The best advice I got was to use a weighted magnet curtain with a cast iron pan. Shower curtain use with shower pan

Thanks Kirsten. The shower dimensions are currently in play while I seesaw between a standard 60" bath, a 54" small tub or a 30" plastic, inflatable kiddie pool. Ha! But more than likely the shower will be around 36"x38" or so. Not big, unless I can find the perfect kiddie pool...then I'll have more room for the shower. :) As for that vanity, we were actually going to do a much smaller vanity, around 65" to leave plenty of open space for that exterior door on the far left. That was before I realized it might be possible to do a skinny/shallow vanity ("might" being the operative word). Meeting with my local cabinet guy tomorrow to try to come to some solution. (In the meantime, I figure Plan B--or C?--will be an IKEA Godmorgon somewhere in the middle...)

Not to be silly or cynical, what are the two people trying to do? If you want two shower positions, each will need the better part of 3' x 4' to be able to shower, wash, etc, without impeding the other. The ideal layout for that is either an double open ended hall style shower or a middle entry with two ends with shower positions. Now, do we also need seats or benches, because thats even more room. If you want a single shower position that two people can use together, 3' x 3' is an absolute minimum for people soaping each other up, and there is very little extra space. In reality, a 4' x 5' would be reasonably functional, however, there would still almost always be one party outside the water flow. A single rainfall head, with additional wands or heads, might work in a smaller shower, but otherwise at least two distinct positions seem called for. With dual rainfalls and wands, a 7+' long shower, 3'+ wide, with a single entry in the middle might function, but a longer one would be better.