Minecraft wiki entry for Chorus_plant, associated text: The smallest possible fully-grown chorus plant (assuming the growth is not obstructed) would have 5 chorus plant blocks.
According to Jeb in the Ask Mojang midsummer special with Jeb, the chorus plant got its name from the soft sound it was supposed to make when it grew. He stated it was not supposed to actually sound like a chorus, but the idea of a forest of chorus plants created a choir together.