FreeGameDev

What is the biggest track you can make that will work in STK? I have seen 3-5 blender units as recommended size, but is 5 a hard limit?

Is there a maximum speed a kart can go- like if it went off a deep ravine, so that it could still land without going into the ground or being "saved" from destruction by the big bird.

Hello

By big what do you mean ? The wideness of the path ? A recommended width is 10 bu (blender unit).
There is no hard limit. Just make sure that is fun to play.
For the length it's between 1min and 1min30.

If by big you mean a large world, the limitation is your computer, your gpu and your talent to optimize things. However I strongly recommend to start with a little track. It is easier. (Keep in mind: making an open world like gta5 took 5 years and a team of 300 talented people).