Floor Space
How much you have determines what speakers you choose
Never have there been so many options in speaker sizes. Newer space age materials used in speaker cone and driver technologies have created speakers that are shallower, flatter and smaller. They can reproduce deeper bass tones than ever before. However the basic principal of moving air in the room is the same. There is no faking air movement or lack thereof with speakers that are simply too small for the room in which they're used. The cone of the speaker is what vibrates the air around it. Since air is what delivers sound to our eardrums, standing too far from the speaker will diminish certain frequencies or tones and the result is a faded sound, like someone calling to you from a distance.
What happens to many people who choose speakers too small for their room is that they might hear some sound just fine, particularly in the midrange. The casualty of speakers too small for the listening room is usually the higher tones. There is no general rule, like a cone size to room area; you have to use basic common sense. A smaller room is from 10'-15'X 10'-15' and is probably suited to smaller micro satellite speakers, especially if you're not sitting too far from them. Larger rooms are around 20' X 20' and very large 30' x 30' and beyond. For larger sized rooms you should at least consider bookshelf sized speakers with woofers in the 6" range or larger. A subwoofer in this sized room could be as big as 12" but no larger. An 8" sub is just fine for rooms in the larger size.
For deep rumbling bass you can feel in your stomach you'll need the extra push a 12" driver will give you. For very large rooms 30' x 30' or beyond, a 12" sub is a must or you might want to consider a 15" sub. Bookshelf speakers are a minimum, but it really comes down to how far your audience will be from the speakers. Very large rooms might benefit from the newer 7.1 systems with added back channel speakers. This setup will bring out the surrounds to parts of the audience that might be too far away to get the impact of the surrounds located on the side walls. No matter what room size, speaker positioning becomes a fine art and it is most important to getting the most out of your choice of speakers.
When speakers are too big for a room you can end up with certain tones overemphasized and the sound doesn't gel together into one sound by the time it's delivered to your ears. This is common in a tiny room with speakers all around that have overly large drivers. Sound is reflected all over the room and cancellations will occur that subtly diminish the dynamic range expected from the speakers.
When deciding on speakers for your audio room consider the amount of space and try to pick speakers that can be placed equal distance from the middle of the listening area. Bookshelf speakers are a fine alternative to larger floor standing speakers, especially if you incorporate a subwoofer to make up for the deepest tones the bookshelf speakers cannot reach.
Don't take the printed specifications of speakers too literally. A pair of mini speakers that claim 50Hz bass response is only telling you the deepest it can possibly reach, not mentioning the decibels it's capable of at that range. Common sense will dictate that the smaller the drivers, the higher the frequencies it will be most comfortable reproducing.