Increasing the size of pipe is more on improving airflow than about enlarging the pipe diameter.
For 1300cc and below vehicle - maximum pipe diameter is up to 2" only
For 2000, to 1500cc vehicle- maximum pipe diameter should be 2-3/8" only.
Mufflerland has these available pipe sizes: 1-1/4", 1-1/2", 1-3/4", 2", 2-3/8", 2-1/2", 3", 4"
Mandrel bend available pipe sizes: 1-1/2", 1-3/4", 2", 2-3/8" and 3"