I was OP from that thread. I ended up settling on 640x360. But I do have somewhat large sprites (64x64) so you may want to go a bit smaller than that.
Note that I have been having some problems in GameMaker with pixels being jaggy when I have color interpolation off, and blurry when its on. I think its most likely because my screen resolution isn't evenly divisible by my game resolution. I'll have to experiment some more. I don't really know what the solution is because different screen will act differently. My game resolution would probably work perfectly on a 1280x720 or 1920x1080 screen. However, my screen is 1600x900.