Trying this out with sketchfab using 2K textures. I was able to generate 8k textures with the updates they did to the pixel processor with 5.2 [SKETCHFAB]63fff6b12c084beeafb290cdc0b9b9cd[/SKETCHFAB]
Also, can you add a grid texture with circles and a per-pixel grid? Maybe the one here with the big R... http://wiki.polycount.net/TextureCoordinates#UV_Map_Grids
search for pixel art tutorials. The hand painted tutorial torch referenced is good too - the same fundamentals apply you're just working at a smaller resolution.
it's a really fun program. Somehow it reminds me of doing pixel art on my Amiga. Is the Marmoset version any different from the previously offered one?