Hi ! :D it´s me again, actually...
I do not know how to make the screen 64x64 without it looking very small, so I can do the 128x128 game with the sprites rescaled?
You can scale the window however you wish, as long as the game still follows the 64x64 grid resolution. If this means making sprites larger (like 4x4 sprite to represent a pixel), then that's fine. Remember though, doing it like that means you'll have to snap everything to the 64x64 grid so you're not drawing at sub-pixel positions.
There will be a category for user to vote on that determines authenticity. This is basically a way for people to say how well you handled using the 64x64 resolution.
Also, what's the tool you're using? I'm sure there will be a more elegant solution than scaling your sprites. Try Googling your tool and "pixel-perfect scaling" or "scaling pixel-art" and I'm sure you'll find a solution. :)
GameMaker has an easy way to do this, you should look into views. You only need one view, set it's width and height to 64x64 and set it's port to whatever size you wish (such as 512x512) although usually it'd be the same size as your window. You can set up views in the room editor. Make sure interpolation is off in the Global Game Settings and it should scale nicely. :)
I used to work for YoYo Games, so I know my way around GameMaker! If you need more help on this you'd be better off searching or asking on the GMC https://forum.yoyogames.com/index.php. In most cases the manual will have good information and examples as well, particularly on views.
In GMS, lookup window_set_size. May I also suggest using display_set_gui_size. If you are using views, just set you view in room and port on screen to 64x64. Best of luck on your entry!
The application surface was added so as to PURPOSELY ALLOW clean sub-pixel rendering for scaled up views, which is an issue is you want to keep 64x64 but scaled up. But seeing as you don't have an application surface in GM8, you simply don't need to worry about it. As long as you set the correct view/port sizes you should be good to go.