15X20 is still pretty small. That is the size of my TV room. 20X25 is a nice room but still not really big. I'm a big speaker fan and I have had big speakers in small rooms and they still sound pretty good. Not a real fan of small speakers in big rooms, they can work but you might end up driving them really hard.
Room size.
I often see folks recommending speakers (or discouraging) certain speakers based on size of the listeners room. While I have a good idea what would be considered a small room (both length and width no more than 15 ft), I am uncertain on the drawing line between a medium/moderate room and a large room. Would a 15 x 20 ft be considered medium, and 20 x 25 a large? Disregarding room height for now.