If you are looking to span that sort of distance, you will need to use PVC Mesh & it will need heavy reinforcing.
The safest way to install is to use a heavy duty galvanised steel cable (I would have thought at least 1 inch dia) with a tensioning bolt.
Have the cable threaded through a sleeve at the top of the banner, and the same again across the bottom. If the banne is being made of more that one piece, you will also need any joins in the banner to run vertically, not horizontally.
The largest we have mounted was for BP, to go between 2 steel towers at a refinery in Germany. I think it measured 9m (w) x 6m (h).
cheers
Jamie