I use a Honeywell HPA300 in my fairly large bedroom. You can buy fairly inexpensive filter replacement kits on amazon. They make smaller ones, but the fan pitch is a bit higher and I don't like that. The 300 is good. I use this 24/7 and a Eufy robovac everyday, and it does wonders for me.

*Editing - I read that ozone can make asthma symptoms worse, so you may want to avoid any air filters that use that.