Your exterior walls will be comprised of either 2x6 or 2x4 studs. 2x6 stud walls allow for more insulation because of their extra depth. Depending on local codes or your desired R-value for the wall, you may use 5 1/2" thick R-20.  Compressing it into a 2x6 cavity will lower the R-value to R-18. For 2x4 stud walls, the choices are R-15 or R-13, both of which are 3-1/2 inches thick.