6" seems to be a pretty good compromise for all sorts of purposes and preferences. It's portable, but still displays quite a bit of text even at slightly larger font sizes, and a 6" device is fairly light, so it can still be held comfortably in one hand. Much bigger and heavier than the Glo, and I could just use a tablet (I prefer e-ink, but size, weight and extremely low battery consumption are also reasons why I read on an e-ink device).
The 6" displays also allow for a device to be in the current price regions of €120 and below. Larger displays cost more and get closer to the prices of the more powerful tablets. The relatively low price of 6" e-ink devices is one of their stronger selling points. Hardcore e-ink fans, which includes many of us here, wouldn't mind paying twice that to get what they prefer, but I don't think the market as a whole is as accepting. A bigger screen also requires a better processor and more RAM, as otherwise pages start to appear more slowly.
The Pyrus mini is only 4.3" and I find it actually more comfortable for reading in bed (laying on my back) than the 6" Glo, but it doesn't have front lights. I'd buy a sub-6" reader with buttons and lights without hesitation. There are probably more people who would get a bigger-than-6" reader than one that's smaller, but I feel that 6" might be the size that makes a solid happy medium.
That said, I like gadgets and would probably consider a reader with a larger display too, but 6.8" (I think that's what the Aura has) isn't enough of a difference for me. 9.7" and we're talking!
There were some 8" ones in the past, but that format seems to have fallen out of favor.