There’s no hard and fast rule here however, i’ve owned several air fryers, and space is often the huge let down. I would recommend buying the largest one you can find. The simple reason is this. If you have a family, or are cooking for two or more people, you will struggle with a small air fryer. If you cook any food that is relatively large such as a schnitzel, you will struggle with a small air fryer. Think of cooking in an air fryer like you would cooking a stir fry. You can do it in small batches, but you’d all be eating at different times. When you look at air fryers, try and picture the food cooking in it to ensure it will fit. In many cases you may just need to resort to doing multiple batches if you have a large family. Don’t let that deter you though, while the units can be reasonably small, they can often have overlapped food and would cook fine with regular shaking of the basket.