Here are a few things to consider:

The room should be pretty much rectangular

This allows having a better control over tuning the audio equipment to obtain the best sound field experience. See the section titled "Speakers placement" for more details.

The room dimension should be at least 15 x 20 feet

This will ensure that we have enough room to put our speakers and other home theater components for optimal performance. 

The ideal room has very little windows and few of them

This will ensure that we can easily control the level of ambient lighting in that room should we want to use a front projection setup.

Keep in mind about noises with regard to your house

Well, unless you are a millionaire with a 20 room house, you will have to consider carefully where you put your home theater so that the other members of your little family can keep living under the same roof. A home theater can produce quite impressive loud levels of sounds!