1 unit equal whatever you choose. My 3D game, George Game 13, of which hasn't been worked on in almost a year, uses two different map scales. The outdoor worlds, which cover several square miles, use a map scale where 8 quants (a unit) is one foot. This makes a mile 42240 quants. To get a building that is 4 stories tall with a 12-foot spacing between each floor, 48 feet tall, I just multiply the size by the scale. It's processed in the same way as converting inches into centimeters or miles into kilometers. With 8 quants making a foot, there are 80 quants in ten feet and 384 quants in 48 feet (as 48*8 = 384). This means I'd make the building 384 quants tall.
For your race track, you use the same thing. Because you'll be using high speeds (assuming past 200 mph), you may want to stick with a map scale of 4 quants to 1 foot or 16 quants to one meter (depending on what units you're most comfortable with). For a track 2 miles long (3.2 kilometers), you'll need the total distance (based on the center of the track) to be 42240 quants in length if using the 4:1 foot map scale or 51200 quants if using the 16:1 meter map scale.
When making something, avoid using the scale_x things in the script (and WED entity settings) as this could pose collision difficulties (collision is outside my realm of knowledge - I haven't worked with it yet). Rather, scale the track in MED itself (but save a backup just in case).
