The BELLA 6qt Pro 90165 is a good small option if you want to spend less. It has a taller but narrower design than the Ninja, which can help it fit into a tight corner. While it has a bigger 6-qt capacity, it has about the same surface area in its basket as the Ninja. However, you can often find it for about half the price, and it makes similarly good-quality air-fried food, so it's a good option for preparing quick, crispy snacks or side dishes.
It preheats and cooks quickly but doesn't maintain as stable a temperature as the Ninja, which makes it harder to accurately follow recipes and cook delicate foods. It's also less versatile since it only has a single fan speed, making it less suitable for methods aside from air frying, like dehydrating. That said, it's an inexpensive, simple option if you're looking for a small air fryer.
If you want something even smaller, you might prefer the Chefman TurboFry RJ38-5-T, whose smaller footprint takes up less of the valuable real estate on your counter. It's a little taller, so keep that in mind if it needs to fit under a cabinet or shelf. This basic model has a single fan speed, so it's best suited for air frying versus other methods like baking. However, it does a decent job of evenly cooking and crisping air-fried foods like fries, chicken wings, and Brussels sprouts, so you might prefer it if your top priority is saving space.