Too often, a purchaser of a piano will start with their budget. They look for the most piano they can get for however much they have to spend. But they really ought to start with the size of their room!
When a person buys a piano according to their budget they could easily buy too much piano or not enough piano. I.e. they will either buy a piano that produces way too much volume for the room where they will be playing it, or conversely, they will end up with a tiny piano with little volume in a large or acoustically sound absorbing room.
Too large of a piano
In the first case, they will constantly "hold back" playing because the volume seems too loud. In this case they develop bad technique and shallow or limited expression in their playing. There seems to be an imbalance between how firmly they strike the keys and the amount of volume they hear. It always sounds like there is more volume coming from the key strike than there should be. Consequently, they hold back their playing . . . they limit their physical manipulation of the keys to a small range, a range much smaller than the piano is capable of. They never explore nor do they experience the full range of that piano, either in its dynamic range nor in its physical abilities for speed, repetition, etc. It's kind of like owning a high priced and powerful sport car, such as a Lamborghini, but never getting it out of first gear when you drive it because you only drive it on down town streets and through parks and school zones. You just don't have the open space to really explore the capabilities of that car.
There is another very strong factor in this situation as well. That is, that the sound has nowhere to go. When you play your piano, the sound should pass by your ears and move away from you to make room for the next sounds you produce on the piano. Those new sounds, harmonies and melodies, should be free from competing sounds. If so, they will be pure and delightful. But where the piano overpowers the room, the previous sounds (chords, harmonies) have nowhere to go. They keep bouncing around that room "mixing" with the next notes and chords you are playing. The harmonies of that beautiful (and expensive) piano get all muddied up and dissonant as sound waves begin to clash with one another. Eventually you find that if you play everything softer, you won't hear the bothersome clashing as much. The piano will sound better to you when always played at its quieter levels. And so, you learn to play it this way. Bad for both technique and interpretation and expression.
It would be better for people to buy less piano, save some money, and feel a direct relationship with the instrument-it's performance, sound, and volume. They would become better piano players, they would develop better playing technique, and they would ultimately enjoy their piano more (and it cost less!)
Too Small of a Piano
On the opposite end of the scale, too small of a piano for a large room will always feel like you can never get the dramatic or explosive expression you want sometimes at the high end of the volume range. You'll find yourself always striking the keys harder than you should, just to get enough volume. Again, this will tend to create bad technique in your playing and in your interpretation of music. You will tend to become one of those "musicians" that always sounds like they're pushing and driving the piece forward, ahead of the tempo. Their music always sounds like it's being manhandled. Even in the gentle subdued, tender passages, they never come across that way. The expression, again, has become compressed into a narrow range; this time at the high volume range of the piano.
The Most Important Thing -- Right Size the Piano for the Room
So here is the most important thing about buying a piano. Size it to the room where you'll be placing it and playing it.
A Real Example:
I have a client who teaches piano performance and she has a 6 foot Baldwin grand. She had it in a tiny room (about 12ft x 12ft with an 8ft ceiling) where she taught her students. Recently, she moved and her piano was placed in the new home on a raised deck open on two sides to a vaulted 2 story ceiling height entry and lots of horizontal space for the sound to move away from the piano. She called me to express her amazement with how much better her piano sounded in the new house! She wondered why that was so. She is playing it now more for her own enjoyment, and not just to prepare lessons for her students.
When you have the right size piano for the room where you will be playing it you will enjoy it more, play it more, and develop your skills and technique more because both the sound quality and the feel of playing your piano will be correct.