If you are not using a compressed air connection, the interactive foam injector size guide below will help you not only select the correct injector nozzle size but also the correct sized foam lance as well. You just need to know the flow and pressue of the system you are using.

An injector using a compressed air connection creates the cleaning foam at the injector itself, this type of system requires a hose with a DN12 diameter to ensure the the foam doesn't collapse back into a liquid state. A wash gun with 1/2" inlet and outlet must also be used.