I find it depends on the hamster and their cage. I've got loads of differently sized hammocks and they all work really well for different animals/cages. For example, if the hamster only has a 60x40cm cage then you may want smaller than if they've got a 100x50cm cage. When I'm making hammocks I tend to look at what space I want to fill and make a hammock that suits that space. I don't think you can really go wrong though as long as the hammocks big enough for the hamster to use and small enough that it fits well in the cage
My biggest hammocks are about 60x40cm, my smallest are about 10x10cm corner hammocks and I've got loads of different sizes in between those extremes too. They all get used fairly often and my pets seem pretty happy regardless of what size hammocks they've got