One year program qualifies you for one year PGWP only. With a 2 year program, you may get up to 3 years of PGWP, in addition to higher additional CRS points for CEC.
Yes, you will have to return back to your country. Unless you would be able to extend your work permit, whch is unlikely. Outside of PGWP, you will need LMIA approval to get another work permit.