At most, karts should be 1 Blender unit wide and 1.5 Blender units long. But really, it's better to go for something slightly smaller.
Since karts are seen up-close a lot, we need to make sure they're detailed and nice to look at. So our polycount requirements have been increased significantly since SuperTuxKart 0.8.1.
- Average number of tris for a kart is approximately 11000 or 12000.
- Absolute maximum number of tries should be ~15000.
Kart-specific Style Guidelines
This info is specific to karts, so it's not on the more general Style page.
Remember that SuperTuxKart is a game for kids. Good characters like Tux, Beastie, Adiumy, etc. are smiling and happy, while evil characters like Nolok are mean and nasty looking. Try not to break the theme of Story Mode too much, though that's not extremely important. Try to avoid suggestive character themes.
Unlike Mario Kart, SuperTuxKart doesn't support sharing karts between different characters, so there's no need to meet a lowest common denominator. Generic karts are rather dull unless it's something iconic like Tux's kart (which has been in the game with more or less the same design since the first release of SuperTuxKart), so go ahead and make a kart you think fits your character. Wilber's kart, for example, is a tube of paint because he's the GIMP mascot. Puffy's kart is a moving aquarium since he's a blowfish.
Add-Ons vs Main
It's possible that your kart will be accepted into the main SuperTuxKart game; however this grows increasingly unlikely as we run out of old characters that need replacement. Please don't be too disappointed if your first kart isn't accepted into the core game—you can always distribute it on addons.supertuxkart.net.