They get bigger than 20' gents!
http://www.theaustralian.com.au/death-of-a-monster/news-story/380314862d53202b600f2f2df8264392
For their size they can move extremely quickly & enter the water virtually without noise, seen it done while fishing for Barra around Normanton. Property owners lose cattle & horses every year & they're only getting bigger as they haven't been shot for 40+ years - no fear of man.