Phoronix article: https://www.phoronix.com/news/Steam-Machines-Frame-2026
Also listed here: https://store.steampowered.com/sale/hardware
Valve has already sent support for the new Steam Controller upstream: https://www.phoronix.com/news/New-Steam-Controller-SDL
I'm not sure what the standard for large vs small hands is, but I haven't had issues with pretty much any controller except the OG Xbox controller:
My kids have no issues with either the Xbox 360 controller or DS4 controller that I have.
I would call it less of an issue (at least in my case), and more of something that became apparent only after going back and experiencing the smaller controller to compare others to.
I really wish there were a modernized controller in the exact form factor of the classic ds2. Like if it had tmr sticks and a better dpad, but was still a wired controller and had 4 shoulder buttons instead of two triggers. Don't get me wrong, triggers have their place, but there are some games that work better with all buttons, which probably partly explains the popularity of hair trigger mods.
DS4 is pretty close.
I like the ds4 - so much so that I want to get a new one and mod it for more modern features. But it's still distinctly larger. It can be kind of surprizing to re-experience just how small and light the ds2 is compared to newer controllers.