Abel, my answer was that it is individual and would simply put another view into it, as I assume this thread is covering both the average joes and the experienced (leaving the racers out though, but they got the same choices just between whether it is an 18 or a 15 to choose).
So for those getting into it more and more, the typical kite setup for the average weights and a bit more (around here at least), end something like a big light race foilkite around 12-15 for marginal winds, a really light no or onestrut 11-12 m2 for a tad more wind, and then some use their normal kites or wavekites around 9-10 m2 and down so they can be used for TT or Waveriding, where others goes all the way and use the light "hydrofoil" kites in the smaller sizes too.
Meaning, the big kites are not prone to dropping if the wind does - a foilkite in the most marginal winds might indeed, but hey, you got no alternative than to NOT go out and have fun, so that is a no brainer - most try anyways, and with practice and avoiding doing mistakes, you can hold a foilkite up in ridicoulously low winds, less than you can ride eventually.
When a tad more wind and you use lighter LEI kites, it is much more pronounced, that these kites will hang easy in less wind than you can ride, and in way less wind than you can get out and up, if wind is onshore.
Once out you can ride in long lulls no problem, eventhough powered on the lighter side, and even if a really big lull, your kite will still hang - these light ones are amazing in this respect, and the reason why everybody choose such kites when over 10 m2.
When too powered they will all flap at some point though, that is one downside, and because as you said a bit higher aspect, they dont turn as fast as even a heavier 9 m2 - and those that do will still have somewhat more on/off feel when powered on the upper end instead of the sweetspot, where you seek a smooth power delivery for most fun.
So no, it is not the "size" in itself that is a game changer here, it is the fact that the typical marginal and light wind and normal wind kites are a lot different going 9 - 12 - 15 m2 (t.ex in 11 - 8 - 6 knots), and for me it is all about choosing the kite that is in its sweetspot as I dont like to ride lightly powered, nor on the upper end.
The lulls doesnt matter regarding which size to choose with these light kites, only difference is many use longer lines knowing there are long lulls, so it is easy to start after "blown" maneuvers, where you have to sit and wait for wind otherwise, or struggle pumping your board and looping your kite a lot, otherwise.
This is not present with the longer lines anymore and you can start again easy, but you dont get overpowered
And this is only the kitechoice, the board and wing and mast makes a MUCH bigger difference than the kites IMO.
My long post was to tell about how some of us automatically make many choices without thinking, and even change on the fly at the beach if needed, in this really "interesting" windrange, used maybe the most as you can not ride anything else (non-hydrofoils), and the weather is brilliant at most spots in this end of the range
The average 10 min wind is almost the best indication, eventhough HUGE differences in the actual power in the wind can occur.
Hand held wind meters will give everyone an idea about the power to be expected on a given day out there, eventually with lots of years experience
But of course, when you are there, you dont need these meters anymore, and in the learning process you use them to learn how much different the measured wind and the real kitepower is ha haa, a bit funny, maybe they ARE useless in fact
Sometimes in the extreme ends though, marginal winds sometimes, and for sure at the upper end when 20-30 knots, they make a lot more sense.