Two years ago I bought my wife a 5.8 qt Cosori. It's what I landed on after doing exhaustive research on the topic. The size is perfect for two people and honestly even for one person I wouldn't go any smaller than that. Unlike most air fryers, it's square-shaped which makes it easier to actually fit more things.
As other people have said, it's really not a fryer, it's a convection toaster oven. The difference is that it's a convection toaster oven with a powerful fan where both the fan and heating element are very close to the food, which I think is key. It does cook things much faster than any oven would, and circulates the air completely around the food.
We've used it nearly every day for two years and it's still going strong. I just recently had them send me a replacement basket because the original one isn't as non-stick as it used to be. What do we use it for? Well, pretty much anything you would use a toaster oven for. Great for fries and tots, frozen pot pies, various meats including bacon and steak...honestly owning it has made me like to buy steak because now I can count on it consistently coming out medium-rare and juicy.
The area it really shines in that no one ever mentions is...reheating pizza. You'll never want to put a slice in the microwave again. I'm not kidding when I say the pizza usually comes out better from the air fried reheat than it was fresh out of the oven (yes, I'm talking both pizzeria and frozen pizzas) because it crisps up the bottom crust so nicely. I seriously order pizza, eat one slice fresh and then put it in the fridge excited to reheat the rest.
At this point it's our MVP kitchen appliance second only to the refrigerator.
EDIT: Oh yeah and this one has a pre-heat function. Pre-heating is very important for getting consistent results and surprisingly that feature is absent on many models.
EDIT 2: I can't believe I forgot to mention that it doesn't make the kitchen get hot at all, great for summer.