William's 3rd source was surprised that ferns could be 3m or 4m high in the carboniferous, and one of the posters above expressed a similar thing.

Actually there are 100s of species of tree fern growing today, widespread in rain forest climates, and one or two other places.
http://en.wikipedia.org/wiki/Cyathea_brownii is said to be the tallest species, but
http://en.wikipedia.org/wiki/Cyathea_medullaris is similar. Both are said to grow up to 20m tall.