Basically, if it physically fits and is aesthetically pleasing to you, you have enough room. Most vertical (upright) pianos are about five feet wide and about two feet deep. Most grands are about five feet wide, and the length varies by model from around five to nine feet. Be sure to add a couple of feet for a player and bench. Piano dealers usually have templates they can lend you, representing different types and sizes of piano, so you can figure your space requirements. See this article in “Piano-Buying Basics” for more information.
If you’re concerned about room acoustics, and fear that the piano might be too large or small for the space acoustically, we suggest you read “Ten Ways to Voice a Room.”