If you want to make your own banners for games in your Steam library the dimensions are 460×215.
Steam Play is Valve's WINE fork. So, in essence, it's a game-specific version of Wine. A cool thing about this is that it gives you a central place to report game-related issues, and those issues are ones they will be interested in fixing. From Valve's point of view I guess it will give them more accurate numbers on how many people actually use Linux for playing games, without it being so distorted by people running the Windows version of Steam through Wine.
It's still early days, but here are some useful links:
If you want to try to figure out why a particular game isn't working you can add
PROTON_LOG=1 %command% to the launch options. It'll dump an error log in your home folder.
You can also enable debugging globally if you want, see: user_settings.sample.py