That depends on your map scale and the speeds involved. Some tracks get to about a mile long as a total and if purely circular the diameter would be 5280/pi quants (about 1680 feet). With a map scale of 8 quants to 1 foot, this means 13440 quants diameter. The faster the speeds, the longer you'll want the track to be. Also, the more course of a map scale you'll want as well (4 quants to 1 foot) the higher the speeds get.

"You level up the fastest and easiest if you do things at your own level and no higher or lower" - useful tip My 2D game - release on Jun 13th; My tutorials