Why should the window size be smaller? I chose that size because i didn't want the screen to scroll, but i still wanted to be able to create variated levels (hard to do on a small screen).
Sorry to jack your thread, man. We're two different people with strong views on the subject. But the above quote is where I'll agree that your game would fit in a smaller window.
Of course, decreasing the playfield causes an inherent problem with all your sprites. They become 28% "bigger" onscreen. They're the same size, but now there's less acreage for which to act in. You'd have to shrink your sprites to about 75% their original size (Construct can do this in the object's properties window).
This isn't necessarily a bad thing. You don't have to change your playfield to accommodate for level variance. The SNES had a resolution of 512x448, yet it still produced gems like Chrono Trigger. If you really look at how modern games are made in HD, you'll notice the extra pixels don't really affect their design too much (16x9 aspect not withstanding). If they were to remake Chrono Trigger on Xbox360 or PS3, They surely wouldn't keep him at 30 pixels tall. Similarly, they wouldn't use 12-point font, would they? The game would be unplayable.
Games are still made with about 480 scanlines in mind, no matter the resolution. The games aren't really getting bigger playfields, they're just getting more detail per square inch. This allows for them to be scaled down to be played as low as VGA resolution and still be readable, playable.
And that's what you should consider with your Asteroids Push game thing. Would your levels fit on a standard 480-line tube TV? If not, you need to rethink sprite sizes, level density, and whitespace (get it? whitespace? aww) accordingly.
While I regard 768-windowed as perfectly fine during the transition to HD, I disagree with why you've chosen to have the high resolution. Deadeye is correct that you should want to make your game as accessible as possible, and he and many others still use equipment that can't support that in windowed mode, or they're unwilling to change their habits just for one program. They are valid points.
I know you put a lot of hard work into those nice graphics and want to show them off. Here's an idea, though. Play with that zoom function to always scale the whole playfield into the current window size. Make your game at a high resolution, and find a way to allow the user to switch window sizes at runtime (Ashley, is that possible?). That way, people who can run your game comfortably at a high resolution can do so, but people who wish to scale it down a bit won't miss out on anything.