The Stream-less annual starter pack includes everything you need for the installation and the first 12 months:
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)
4 x Stream-less cartridges (replace every 3 months)
1 x Stream-less flexible hose and connector (for unusual installations)
As per description above.
The size you require is measured by the diameter of the thread of your existing bottle trap.