Lane width really depends on how many cars you plan to run at one time and how tough the track is. Also, remember that even though the track is smaller, lanes that are farther away will feel narrower than ones up front.
My suggestion is 6-8 feet, depending on the criteria above. Corner lanes or switchbacks can be narrower if need be if you don't plan on having more than 2-3 cars on the track at a time but passing will be tough.
Also, try to keep any big jumps you put in closer to where you stand to drive, it will help keep crashes to a minimum. Then make the back sections easier.
People have a tendency to put straight away sections in the front and triple jumps in the back when it really should be the other way around.
Have fun with your track!!!