Is this a track for fun in your yard?? Or one for organized racing?
Shoot for a minimum of 8ft wide lanes.
For Organized racing-
Electric 100-120ft back straight. For Nitro a little longer.
Jumps should be fun!!! NOT Car breakers. if you have multiple jumps (doubles or Triples) adjust the distance (timing) to make them challenging but not impossible to complete sucessfully.
Have fun with the layout, but don't let the difficulty of the track be the main determing factor in who wins the race instead of the drivers set up and driving skills.