Portion - golfball sized.
Subwassertang is a livewort that thrives in the low light conditions that are typical in a shrimp tank. In my experience Subwassertang does not attach to wood or rock but sinks and will form a nice bush as it grows.
It is also sometimes called Round Pellia. The scientific name is Lomariopsis sp. It may be Lomariospsis lineata or it may be a different species.