This is a somewhat silly design exercise, brought about in part because my wife Diane was a little upset to learn that our 371cm instrument was not the largest in the world (it actually never had been). There had recently been built an eighteen foot instrument that was strung with plain wire only. That is, there were no wrapped bass strings.
This got me thinking about very long plain wire scales, and I was curious to know how long a plain wire scale would need to be to actually have lower inharmonicity than the R-371. I did some scales and rough cartoons to find that the instrument would need to be about 21 feet long, and even then the strings were only slightly less stiff than the 12 foot scale. So one can see that see that we are entering an area of greatly diminished returns.
Of course if you are only interested in record breaking, and not in how an instrument sounds or rather how long it sounds, then it is relatively easy to build the longest piano. One only needs to refer to the 21 foot long piano recently unveiled in Europe that is actually several smaller pianos glued together.