More of an optimization thing. I'm pretty sure that I've done 16384x16384 textures for kart skins back when the template had all body parts on the same template, but you don't get much of a difference once you go past 4096x4096. Why use additional resources if you don't have to? Not everyone has a great computer (and sometimes it makes optimization difficult). I've always strive to be able to run the track on a laptop, even if I'm getting 200+ fps on the development PC. If you are using atlases/packing multiple textures onto the same image, yes - 8k or 16k may be reasonable. However, if you are doing single textures (for example the GoPro logo) you will not need to use a 4k resolution image. While the amount of collidalble objects is usually the main culprit for low framerates, your texture size may be a factor in low framerates.
As of now, since nobody has tested the track, we can't be certain. I've created tracks that are 150k+ polygons but did not have huge collision calculations, so they ran well on low end systems.
Edit: after reading through the thread again, Lauzzzn said that 4096 is the max texture size. I can check that later today, but that might be the maximum size.
