I have a 64gb with a large sd card. My recommendation is to get a 256gb. You will feel space starved with installing different versions of Proton, the shader cache(?), and the emulated Window’s user folders.
If you get the 64gb you basically have to get an sd card. If you get bigger then you might not ever have to if you play relatively small games and you don’t mind managing installed games.
I don’t think that’s how it works, but I’m too American to be certain.