I never really understood the benefit of 8 inch (I have had one). It is often too big for the pockets and too small for some productivity work. The non-eink screen is also not perfect for ereaders. Only as a handheld gaming device is it perfect, if the games are available. Perhaps a 4:3 8 inch like the Ipad Mini for browsing etc, but you would miss the Apple apps I guess.
I had an 8 inch for awhile. It was frustrating to use. Maybe with a pen, but generally, not great.
I feel like around 8-9 is the minimum for the current windows UI. Nobody makes those though, and you can still technically fit that in a large jacket pocket.
Amazon and some reader focused android devices make 8.9. Perhaps there's a market there in a "just one handable" windows device, that is also just big enough to be pleasant to use with windows 10. Pair it with a pen, and that might be pretty solid.
(My dream tiny windows device would be 8.9, have a good pen, and have laptop ram slots so you can max out the chipset's performance. Two microsd slots is something you never see, and that'd be kind of handy in a small form factor, could give it decent storage - either than or m2, if it can be fit - like the ram, there might be space limitations)
I did as you say, use my eight inch with one of those telescoping blutooth gaming pads. That wasn't bad for mobile gaming, but at that size, and power, battery life for gaming isn't great.
For smaller devices you need smaller batteries. Which means cpu's with lower power draw. None of which work well with gaming apart from really light stuff (part of why I always dream of ram slots when I think of mini-pc's and small tablets, they use crap all power).