A diffuser for a large room is not the same as one of the regular-sized models. They have a larger capacity tank to make sure they can fill the entire room with the essential oil fragrance or humidity without having to fill it up all the time.
I had a look at some of the most popular diffusers that are large enough to fill the whole room with their healing smell and other great benefits that they offer.
I found these 5 have the highest ratings and best comments on many consumer websites and some forums I was looking at.
Urpower Aromatherapy humidifier
VisTsing Essential Oil diffuser
Diffuser aromatherapy and humidifier
1500 Ml Aroma diffuser
Aroma Therapy Machine
It seems on the picture that these diffusers for large rooms are very big, but the picture makes it look that way.
After all a diffuser for a large room needs to be able to fill the whole room.