The industry standard, for trickle filters, is 2.2gals of 1.5" bio-balls per 100gal (US) of system with a moderate bio-load.
!.5" bio-balls have 160 sq/ft of surface area for bacteria to adhere to. Approx. 3.5 gals of this bio-media will suffice. Switching to other materials will reduce the amount you'd need by half. Plastic pot scrubbers and floor-buffer white stripping pads contain 370sq/ft of surface area.
Here's a table to compare materials: