According to wikipedia the following CPUs were built with that:
Which i think of as more than sufficient to finally being able to implement something like SCED, WAM, CHERI, Applecore, whatever in whichever way one is able to wrap his brains around it. Asynchronous, fully static?
(with MEMS(for sensing)) from scratch?
I want it all! I want it NOW!
under R4300i which mentions 45mm² for the die.
Also mentioned here  https://bits-chips.nl/artikel/small-series-of-chips-profitab...
are 0.25-micron to be released this year, with 190nm and smaller on the roadmap.
Which leads us to  https://en.wikipedia.org/wiki/250_nanometer at least.
Maybe not comparable in die size for all the chips mentioned there, i don't care so much, because i don't want to clone or emulate them. I want to go simpler. Rebranch from the 70ies so to speak, to take all the roads not taken since then. Just to see what's there :-)
0: The main reason to use a rectangular die is that they tesselate better to fit many dies per wafer.