Hm, don't know if there is a definition of 'specimen' size plant, but in my mind, it's what your blooming size plant wants to be when it grows up. It would be a lot bigger than just a "blooming size" plant. To me a specimen plant is either in bloom or will bloom on multiple stems within the year I bought it. (not that I buy them)
If you go to buy a real 'specimen' at a show, (and they won't often sell them) you pay big bucks! Last one I recall seeing for sale was at the Fender's going out of business sale. They had regular blooming size plants for $20 to $30 bucks but this big specimen was $250. It was also "Sold".
If you figure they'd have to sell 10 divisions at $25 each to get that much out of this plant, they did ok and saved themselves all that pesky dividing, too.
You see fabulous specimen plants at botanical gardens too, of course. Here's Jim standing beside what we called "that little Bulbo" at Selby Gardens.
I think my noid Catt is now big enough to call a specimen plant, since it blooms on 3 or 4 stems at once. But when I first got it, it was in bloom but by no means a specimen.