When a polyp is open the circle represents that polyps full size, it is the Diameter of that polyp.
For example, People Eaters are one of the larger polyps, and fit closest to the H, so all People Eaters would be H scale. The C scale would be for large polyps like the Nuclear Greens. Sun Polyps are the largest, so they would be apart of the A Scale (I know its not in that graphic).
I did not try to make a graphic that was completely accurate because I figured there was something out there like this already. The letters I picked for the scale mean nothing. I just took what I remember from toy trains and how they are categorized. Z being the smallest scale toy trains they make.
I think you get the idea, if someone was to tell me they have some H scale polyps they want to trade, I at least have an idea of the size. There have been a few times when I ordered some Zoanthids online, and when they come in, they were T scale, and I was expecting something in the H scale range.
This is not something that is to be exact, just a general size of that type of polyp. I understand that my OPE might be smaller in Diameter then my buddys OPEs because of different flow and lighting, but they would always be catagorized into the same scale (the max a polyp could be is the scale it should be put into).