I have the 36 x 60 Underscore but I’m only 5’5.” If your DH is tall probably he’d be more comfy in the 66” long tub. It’s a great tub, enjoy!

Two comments about your bath layout:

- I don’t like my back to the door while in the bathtub. If that bothers you, put the bath filler on the shower side, so that you‘re facing the shower when in the tub.

- if you eliminate the shelves at the end of the tub, you can make your shower that much larger.

I've listed the bottom basin area for both the 60" and 66" taken from the specs on the Kohler website and even though there's 6" difference in size, the bottom basin area is only 2 1/2" difference. What strange is the bottom basin width isn't the same size. Not sure if the specs are correct or not since I would have thought that the inside width would be the same.

If you look at Kallista Perfect tub which is 60"x32", the inside dims are larger then the Underscore 66"x32" which seems strange.

When looking at different tubs look at the bottom basin size since that's what's going to matter for comfort.

Kohler Underscore 60"x32" - Basin Area Bottom - 42"x23 1/2"

Kohler Underscore 66"x32" - Basin Area Bottom - 44 1/2"x22 5/16"

Kallista Perfect 60"x32" - Basin Area Bottom - 45 13/16"x 24 3/4"

Thank you @AJCN & @jjb 61! I think I'll get the 66, but now I'm a little worried about whether I should get the 32" width or 36" width. So much of the plumbing stuff feels like a mystery to me!

The shelves are probably expendable, especially now that we've added a shallow linen closet in the hallway to the master. But I'm basically trying to pack in storage everywhere since it's a small house. Our architect says we can still fit the shelves with the 66" tub.

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Have your cabinet builder make you a large cabinet for over the toilet. Ours goes wall-to-wall and it's taller than a typical one you would buy retail. That's a good place to add storage. It's just a personal thing, but I don't like open storage because of having to dust things, especially if the storage is mainly holding decorative things. For me, I'd probably forgo the storage shelves and make the shower wider by 12" -- a whole foot wider shower would be super comfy, especially since you want a bench which takes up a lot of the footprint of the shower.

Editing to add:

In the place where the shelves used to be, put a beautiful eye-catching print that you love. You'll see it as soon as you enter the bathroom, the art will reflect in the mirror, and you'll see it when you are taking a shower.

My husband is 6'3" and loves a good soak in the tub. I installed a 66" trop in soaker tub (Hydrosystems but considered a Kohler) and the size was perfect for him. I think 60" might have been a bit too small for comfort. The 60" would have worked fine for me at 5'6" but the 66" was also comfortable. (We have since moved from that house.)

Regarding the width, is there a plumbing showroom where you could sit in an Underscore and the other one that was recommended above? A lot of times it's hard to find a showroom with the tubs you want to try out.

Mine is 36" wide, but I just got into my Underscore, and I think the 32" would be pretty much just as comfortable; but I'm a small person. If your hubby is a bigger guy, he would probably appreciate the extra width.

Looking at your drawing, it looks like you have 41.5" of width available for tub + decking. If you do minimal width decking you can fit a 36" if you want to. We have 1.5" of decking along that back wall, 6" at either end, and 6" on the front edge. That's plenty for me for putting soap and shampoo and such. In your case, you would need to shrink the front edge to 3.5" of decking along the front edge of the tub. So it would be a trade-off, more width inside the tub, or more width of decking to put stuff.

I have the 60” Underscore and am 5’9. I find it too short and the angle of the back is awkward. Not much of a bath person so it isn’t a problem for me, but if I did prefer a bath I would be unhappy with the choice.

I am about to install the kohler underscore. Did you choose that tub and dd you drop in or undermount?