The MTD-300 and 600 are 12" long and 24" long. These are hollow 2.6" diameter tubes with a rubber membrane wrapped around the outside. These must be filled with rocks or gravel in order to be weighted down. 

The MTD-300 and 600 come with an end plug and a multi-barb adapter so you can connect with various hose sizes from 3/8", 1/2", 5/8" or 1.

MTD-300: Matala Tube Diffuser 12" non-weighted w/ multi-barb and plug

MTD-600: Matala Tube Diffuser 24" non-weighted w/ multi-barb and plug