I'd like to use GOG Galaxy to only download and update the games I've bought at the time I'm going to start playing a game. I do not want it running as a background process, and I do not want it to connect to the net without my OK. Is that possible? At the moment, I'm manually downloading the game(s).

When I ran GOG Galaxy, its display screens were larger than the borders of my monitor. Are there settings to change the size of its screens to fit the width/height of the monitor?

P.S.: I forgot to ask, can GOG Galaxy install the game to a folder in a directory of my choice? Windows defaults to using locations of its own choosing. I prefer to install games in folders such as C:\gamename

1 = yes assuming you have Windows 10... in 7 or 11 there are defaults that can be harder to find\ set

2 = Windows sets the size of your desktop screen and may need to be looked at because Microsoft loves replacing drivers with their own crap which then doesn't work as well... if your screen is set correct in Windows then Galaxy will use that default size

3 = yes I have all my games in G: Galaxy but be aware that the download or temp folder should NOT be sent to the same place as the install folder i.e, G galaxy\ temp and G galaxy\ Games is correct because you don't want temp folders getting synced up to the cloud with your game saves by mistake

By default, GOG Galaxy starts up when Windows starts, but you can disable that in the settings.

If you can't grab the top of the window to move it, then place the cursor over a corner to resize it, I don't think there's much you can do besides contact support.
There's no setting for the window size and I don't know where that information is stored in the configuration files. Also, the Windows shortcut Run: Normal window/Maximized setting has no effect and neither does the Alt+Spacebar keyboard shortcut.

You can set a default game installation folder of your choice, and on top of that you're prompted for the desired installation folder every time you install a game.

Just diable GOG and you play your games without any updates, It's a very friendly launcher towards the customers.

There are 2 background services installed by Galaxy, that if found by a game, will be started automatically, even if Galaxy is closed.
These services will be stopped automatically after you exit the game.

Those are needed mainly for online stuff like multiplayer, or to make the game contact the main developer site, like happens with "Dying Light".

Except for this, where there is no "official" control at all, you can disable pretty much everything from the Galaxy client (cloud saves, time tracking, achievements, auto-updates, etc... etc...).