Is seaweed the largest kind of green algae?

Question

Um, I believe they are not, but am I wrong?

Answer 1

A type of kelp is the largest algae.  While kelp is considered a type of seaweed, it's a brown algae, rather than a green algae.  

But one of the largest of the green algae is also a "seaweed" called water lettuce (Ulva sp.) which form large green sheets.

There's another type of red algae (coralline) which grows as a hard crust on rocks, so I suppose that might be even bigger if the rock it was growing on was large enough.  The first two are more of what you think of as algae with most of the organism floating rather than being attached to a surface.