They grow up to about 30cm(12in) and its pretty much their regular behaviour. They just float around near the kelp all day. If you want to see some REAL GOOD footage try this site. http://www.aliensofthesea.com/ . The cover has a Leafy Seadragon on it which is a very close relation of the weedy but only found in a small area of Southern Australia. Has a few clips on the site with both weedys and leafys. Check out his camera gear if you just bought the latest and greatest and feel very inadequate. Hate to be in a current with that sucker.
The offer is open to anyone coming through Sydney if you want to go see one or maybe 2 let me know.

Matt