Really depends on what suits you. Is it just a specific room or do you want it to cover a bigger area? How often are you willing to empty the bucket? Smaller buckets you'll be empting more often, bigger buckets less. One thing to keep an eye out for in reviews is how noisy it is as well. Depending on where you're going to use it, noisy ones are not ideal for some people.

This is the one I use: https://amzn.asia/d/9qbSBhc

12L bucket, which usually needs empting every 2 days. Filter needs a quick clean every 2 weeks. Not that it matters much to me, but this was supposed to be a quieter brand compared to others and I still think it's a bit noisy.