Few of my games feature pixel art, but of those that do (finished or not) I think most use 8x8 tiles. :P Okay, okay, that's hardly useful, but seriously, from what I've seen around here, hardly anyone does non-isometric tiles larger than 32x32, and that's strange, because 64x64 would be SNES-era graphics. As for resizing them, downscaling might just be easier than upscaling, but either way it's not going to work without lots of manual corrections. Except... Maybe vector art would render better in various sizes? Inkscape for instance does its own antialiasing on export.

My own art skills are limited, so take these answers with a grain of salt, but for what it's worth:

1. Any artist worth their salt should be able to follow an existing style. Maybe not perfectly, but enough to avoid clashes.
2. Your own art has plenty of style. That means enough cues for someone else to follow, and not leave you behind.
3. As for the workflow, it's best to discuss that with the artist once you're in negotiations with one.
4. Likewise for the payment, but in my experience art commissions of any kind are paid per piece. More detailed pieces cost more, but still a fixed price and not per hour.
5. Assuming the artist insist on doing their thing (but see point 1), they'll still want to see examples of what you want, to know what to avoid if nothing else. :P

Hope this helps.

Best of luck with your new business! Looks good so far. But I sure hope Cubebrush doesn't become the only destination for game developers looking to trade art assets. A place to buy or sell high-quality work for suitable prices, that's welcome.

Hi! I'm using this piece as the main theme in my game Lost in the Jungle (along with two others, but I don't want to spam). Thank you for making it.