graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
graphics.PreferredBackBufferWidth = 600; // only new line!
graphics.PreferredBackBufferHeight = 300; // only new line!
// graphics.ApplyChanges(); // not needed - but tried without success!
MonoGame doesn't do anything with graphics.PreferredBackBufferWidth or graphics.PreferredBackBufferHeight – if you used their 800×480 defaults in your XNA game, you'll find that they're zeroed in MonoGame and setting them to anything else has no effect on the game's behavior.
This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)