as they said make it any size you like if you just having a spin.
Shoot for 10 mtrs wide in the strights and 14m in the corners if you can to get a feel of a normal track.
Best size would be around the 330mt at the pole line, ( which is 1m on the inside of the track)
Any more and its too long/straight and any shorter and you may as well ride a small circle.
If you struggle to figure it out, find the biggest car you can and hook up the longest trailer or caravan you can find. Drive its around with the car at left turn half circle, then strainghten up, then left turn and so on. This should give you ruffly the right curve shapes, a feel of how flat the ground is or will need to be.
The straight you can turn left when you get it out of 2nd gear in the car train
prity rough and dirty but be bloody clost to perfect track shape and size - good luck and have a blast