Comment by enragedcacti

As the OP talks about a bit (see (math) in the "On Weakness" section), the things that really matter are:

1. the rate at which clean air is replace with dirty air, the ventilation half-life (e.g. steady state from an outside draft, bursts from cooking)

2. the rate at which the purifier extracts particles (CADR)

3. your personal tolerance for particles.

4. (unstated in the OP) your tolerance for noise level.

Ikea arrives at that size through some form of that math, but if you live in a less polluted area, have a well sealed home, or just have a higher tolerance then it could absolutely be suitable for a larger room.

You can buy air quality sensors to test this or purchase a purifier with one built in, such as the Starkvind from Ikea. it can automatically adjust the speed to satisfy some level of pm2.5 particles (I'm not sure what that level is because I don't have it connected to anything smart). I have this in my bedroom and find that the vast majority of the time it stays on setting 1 or 2.