Just larger than your sleeping bag and mattress, bivies are typically constructed of waterproof or water-resistant materials to shield against wind and precipitation
Its one of the highest rated backpacks and I can definitely see why
If youre new to backpacking, generally youll want to figure out what season and temperature youll be packing for (winter, on average, needs more capacity than summer), how many days youll be out (how much food/fuel to bring), what the water situation is (is there water where youre going, is it potable or filterable, how much should you carry, etc.), what the animal situation is (are there bears, and bear lockers, do you have to hang your food at night, etc.), and what the bathroom situation is
Are the seats matching or is one a male and one a female seat if so how do you tell the difference
The more vinegar added, the stronger the effect