What is the right screen resolution for movies on this site? I don't know if I should use 1024x768 or 1280x1024, since newgrounds doesn't list which resolution is right to choose. Is there a difference between these two resolution sizes?
I really don't know what to put on here.
I'm not entirely sure what you're asking, but if you mean what dimensions to use for movies submitted to the site, there really isn't a standard, though the max 16:9 size for .swf movies is 910x512, anything bigger will break out of the window (I think that still happens at least, might've been disabled for movies), and it's best to have video files be 1920x1080, though any dimensions will do because the video player will fit it into a 16:9 rectangle anyways.
I know there's been a page mentioning recommended standard sizes before... but can't seem to find it now. It might have been removed as standards keep changing, and screens grow both larger and smaller (with many users opting for mobile).
As artistunknown mentions the aspect ratio is generally more important than overall resolution, since the resolution can be scaled up or down depending on the device. For any format that doesn't inherit the width of the layout (like Flash), then a size smaller than the typical screen is better, since it won't break the layout, or be larger than the confines of the screen, for users with smaller screen resolutions. For regular movie files it'll scale depending on how the user views it, so I'd say just focus on aspect ratio and use the best resolution possible, or a good balance between resolution/filesize.
The ones you mention aren't widescreen formats btw, which is currently the most widely used. Either one you choose will work fine though. I'm on 1280x1024 myself (that's 4:3) and prefer this format, though I know I'm the oddity there. Most modern monitors seem to be 16:9.
