Server https://github.com/LizardByte/Sunshine

Client https://moonlight-stream.org/

The game/screen is captured and streamed to the client, and screen size doesn’t matter. I tried playing Elden Ring on my phone and it worked 😂 although touch controls were shit

best way I've seen is to set your game to Windowed mode, and then in game set the resolution of the game to run at the client's. Not super if you frequently move around, but if you always play one game from one client, at least it's always a "per game" setting.

If you want a real challenge and want to have it a very specific way... a very fun nerd project is setting up VMs with GPUs that can run Steam, and those can always have a specific resolution. I did that for a while, it was a ton of fun, and a ton of wasted time because I just wanted to do it

I use Steam Link on the Deck to do this often. Personally I just leave it in 16:9- the Deck just displays horizontal black bars to get from 16:10 to 16:9. For me that's just part of the Deck experience because even a ton of games I play locally are 16:9 only, or even 4:3.

You can choose to match the resolution as well. On your host PC go to steam -> settings -> remote play -> [advanced host options] and look for something like "Match desktop resolution to client".

It also depends on what you are doing. I find that more resource-intensive games are often better streaming- better battery life with less heat and fan noise on the Deck. So changing the individual game settings to 1200x800 works well.

I don't use that just to access files though. I have several folders on my desktop set up as SMB shares and installed another file explorer (Nautilus) to access it. I mostly use it to move around PS2, GameCube, and Wii roms- everything else is either just permanently on my SD card or I'm able to regularly install via Steam.

For non-Steam game saves, I set up SyncThing to synchronize across devices.

Another option may be to try using a dummy plug to fake a monitor with whatever resolution and aspect ratio you need. There are scripts that work with the sunshine/moonlight combo already listed that will switch your primary to the dummy plug when connected and switch back on disconnect.