Nukkus: You guys do realize that even if it was possible to set 1080p as a resolution, the way this game is rendered it would be too small to see what's going on. Jazz would look like an ant.
cikame: Specifically i'm looking to set the game at 800x450, which is apparently possible thanks to JJ2+, i just don't know how.
Your GPU needs to allow it and since it's not a common resolution you need to add it as a custom resolution in your GPU driver settings, then JJ2+ should list it.
You also need to make sure your GPU does scaling and not your display (since your display might refuse an uncommon resolution like that).
Though if you're not even seeing 800x600 it sounds like JJ2+ is installed incorrectly.