PlayerPreferences.cs

using Godot;

public class PlayerPreferences : Node
{
    private static ConfigFile Cfg = new ConfigFile();

    [Export] private static string Filename = "player_preferences.cfg";
    
    private static string Filepath = "user://" + Filename;

    public override void _Ready()
    {
        LoadOrSave();
    }

    private static void LoadOrSave()
    {
        switch (Cfg.Load(Filepath))
        {
            case Error.Ok:
                Load();
                break;
            default:
                Save();
                break;
        }
    }

    private static void Save()
    {
        Cfg.SetValue("GameWindow", "Width", OS.WindowSize.x);
        Cfg.SetValue("GameWindow", "Height", OS.WindowSize.y);

        switch (Cfg.Save(Filepath))
        {
            case Error.Ok:
                GD.Print("Saved!");
                break;
            default:
                GD.PushError("Could not save file!");
                break;
        }
    }

    private static void Load()
    {
        var ScreenSize = new Vector2((float) Cfg.GetValue("GameWindow", "Width", OS.WindowSize.x),
            (float) Cfg.GetValue("GameWindow", "Height", OS.WindowSize.y));

        OS.WindowSize = ScreenSize;
    }
}

PlayerPreferences.fs

open Godot

type PlayerPreferences() =
    inherit Node()

    let cfg = new ConfigFile()

    [<Export>]
    member val filename = "player_preferences.cfg" with get, set

    member this.pathToFile = "user://" + this.filename

    override this._Ready() =
        this.loadOrSave ()

    member this.loadOrSave() =
        match cfg.Load(this.pathToFile) with
        | Error.Ok ->
            this.Load()
        | _error ->
            this.Save()

    member this.Save() =
        cfg.SetValue("GameWindow", "Width", OS.WindowSize.x)
        cfg.SetValue("GameWindow", "Height", OS.WindowSize.y)

        match cfg.Save(this.pathToFile) with
        | Error.Ok ->
            GD.Print(sprintf "Saved data to: %s/%s" (OS.GetUserDataDir()) this.filename)
        | _ ->
            GD.PushError(sprintf "Could not save file at %s/%s" (OS.GetUserDataDir()) this.filename)

    member this.Load() =
        let screenSize =
            Vector2
                (cfg.GetValue("GameWindow", "Width", 1440.0f) :?> float32,
                 cfg.GetValue("GameWindow", "Height", 900.0f) :?> float32)

        OS.WindowSize <- screenSize