We purchased a Victorian and Albert 60" Shropshire slipper tub. I am average height at 5.5 1/2 inches. The interior measurement of my tub is 43" (flat area for bootie and legs). My legs extend strait and the length is perfect for me to lean back on the slipper part. It is very relaxing to recline. Sitting up strait did not feel like there was no support to the back because the slope is not like a chaise lounge recline. I will be buying a tacky inflatable suction cup pillow though. My head seemed to go back further than I may want. Who knows. The Volcanic limestone is supposed to absorb and retain heat very well so, I may just prune up beyond recognition and stay in the tub forever. I am not a big bather only due to our cultured marble jetted tub that takes a lot of water to fill (it's really wide) and the jets don't work. I sat in a few free standing tubs at the plumbing supply and all were good, just the slipper gave you that little extra luxuriousness to just lean back and soak. And it is just so darn elegant. The one drawback is you are in a very awkward position to try to put your arms on the rim. It's not cut out for that, due to the slipper shape.
I had room in the 66 inch tub (461/2 interior). My husband, who is only 5.10" had to bend his knees. From what I can recall (this was back in early Oct), where his knees came up to was still below the overflow so they would not be sticking out of the water. Your husband is tall, and his knees may stick out. That said, you have 44" interior space. Mark a 44" area on the floor and have your spouse sit in the space. See how far he has to bend his knees.

Very helpful. We're expecting some bent knees given the tub size...but we're just trying to shoot for the best situation we can manage given constraints. He's been making due in a shallow 60" tub for years...so we're hoping for the best. Thanks!