If you're hammocking Gaz get at least a 3x3m if not bigger. If you're carrying it on your back look for a lightweight nylon version, but if you're in a canoe or camping near to your car then a polycotton model is nice to have. Hammocks tend to be about 2.5m long if not more, you need a good overhang with a tarp for when the rain's coming down sideways.
If you're on the deck then you can get away with something a lot smaller like an issue basha.
I have the aussie hootchie, DD 3x3, SOTP big tarp, Hennessy Hex and UK Issue lightweight basha, all good bits of kit.