I think I just got my answer!!!
I just told my boss that I spent the evening last night looking for something, ANYTHING, on line that would answer my question, and I told him I'm looking to find out how big to make the square, when the vertical measurement would end up at 3" (unfinished). He said, "Multiply 1.5 (or half of the height) by 1.414" - it came up to 2.121. Or 2 1/8", basically. Then he said maybe it should be 3 divided by 1.414 - which comes out to 2.121641, or basically the same thing. He said that 1.414 is "the tangent of 45 degrees". (Whatever that means!!!!) It's evidently a trigonometry equation, which I didn't get to in college!!!!! LOL! Not that I'd remember it all these years later, anyway!
So, I drew a square, 2 1/8", and measured it corner to corner, and it's 3"!!!