Thank you! :)
For the small screen issue, you can fix it by going into the Settings and toggling fullscreen on, which will scale the game to the maximum resolution.
As for the issue with the grey visuals, it's a strange behavior I found while testing Pumpice on the Steam Deck, both with the Linux version and through Proton. Unfortunately, I'm unsure what the cause could be, as the game works fine on other hardwares & OS. It could be a fault of mine that I missed, or an issue with the Steam Deck and Godot, but I can't say for sure. For now, I don't have any fixes available, so if your only option is with the Steam Deck, I'm sorry the game isn't playable currently :( I'll try to fix the problem when I can submit an issue, or if I find any related information, but for now I'll mention that the game is unplayable on Steam Deck in the description.