I'm trying to figure out the correct size of a jetted tub for my master bathroom. I'm attaching a picture of the floor plan for the bathroom and would love input. The tub space is approx 7'10" x 42". I love the Kohler Underscore Bubblemassage tub and was hoping to buy it from a particular store, but they only carry the Underscore in 72x42 (too big?) and 66x36 (too small?), both of which I think are the wrong sizes for my space.

The Underscore is available elsewhere in a 72x36 size, which I think is the correct size. I would love to purchase from the store mentioned above as they have great prices, but not so great that I should sacrifice the correct tub size just to get a good deal (though it is tempting me :) ).

Any input on what the correct tub size for my space is? I'm specifically looking for a jetted tub, which I think rules out freestanding tubs (too bad, since I love them) and therefore an undermount is the next best thing in my mind.

Thanks in advance.