I think 60% is a bit too much (unless you live somwhere really hot). I find 40-50% shad cloth to be perfect for carnivorous plants. It cools downs the greenhouse significantly while still letting in enough light for the plants to use.
As for the size a 6x8 shade cloth is what you would need to cover the entire greenhouse roof. It might actually have to be a bit wider than 8 feet since the roof goes up a bit, so I think 6x9 would be perfect.