Yup, agreed, the UK discussion and pictures add a different perspective on this (pasted below).
I don't think the kites need to be marked for their exact size, it's all how they fly relative to each other. That said, it would be nice if there was a standard measurement of size as I find my slingshots tend to be smaller than some of the other kites flown in my area. Somebody will lay out an 11 that looks bigger than my 14 and then brag about what a great low end the kite has! Then again, even with standard sizes it would be arbitrary due to what the company does with the canopy shape & bridle.
Here is what I have learned in my exhaustive 7 year kite study and personal demo quest.
Kites that have "a great high end" usually have a shitty low end.
Kites that have "a great low end" are usually scary when you depower them in high winds.
All fancy design attributes aside, more fabric = more power.
People that say "I only ride my 6M, bro." have a much bigger board than you, weigh as much as a squirrel, and live in a very windy place.
Per this UK thread, this is an 8/10/12 laid out flat;
Good discussion.