The Stream-less Installation Pack includes everything you need to install:
1 x Stream-less 'L' bend
1 x Stream-less one-way valve (replace every 12 months)
1 x Stream-less one-way valve holder (threaded to screw into the 'L' bend)
1 x Stream-less cartridge (replace every 3 months)
1 x Stream-less flexible hose and connector (for unusual installations)
Full kit as described above.
The size you require is measured by the diameter of the thread of your existing bottle trap.