1 Depends on the road. N scale is 1/160th so measure your road and divide by 160. But roads vary from alleys to highways to super freeways. There is no typical road.
2 There are NMRA standards for tunnel openings. Most of the tunnel openings you can get at your hobby shop will be correct, but you can get the exact measurments either with an NMRA guage or from www.NMRA.org.