How big a tank do I actually need?

For a single bedroom, a 2.5L–4L unit will typically run a full night on low. For an open-plan living area or a room over ~400 sq ft, the consensus in r/Humidifiers leans toward 6L+ ultrasonic or an evaporative whole-room model. Tanks over 6 gallons (like the Levoit 6000S) start moving into whole-home territory and add wheels and a fill hose for a reason.