I think it could be interesting to use the shrink property here (it's in the stretch options). It downscales the rendering by the factor you put there, so that way you can set the window size to something like 256x256, and then a shrink of 4 will make the game still render at 64x64 (256/4=64). That way you can have the game window at a higher default size, but you will still always have the 64x64 internal resolution.