Thanks for the manual image; that definitely explains some stuff I didn't notice in-game. I don't think I would have ever found Lucid Mode on my own, heh. That explains the big box on the screen, which I had thought that was actually a placeholder for a picture.
Regarding the header, Mesen has a header editor (Debug -> Edit iNES Header) that you can use to set various properties, including an 8 KiB size. Sizes smaller than 16 KiB are a little challenging to do manually because they require a different, more complicated format added in NES 2.0. That'd probably reduce your emulator and flash cart compatibility, though, since I'd bet some emulators and PowerPak (maybe even Everdrive) won't handle this other size format.