What is a good length for the statement of purpose in a U.S. PhD application? Certain schools give you specific details about their statement of purpose. For example a maximum of a certain number of words, double spaced, two pages maximum, etc. If a school does not provide any of the above-mentioned details, what would be a good lenght in terms of number of words to engage the admission committee? Is a single space statement of purpose too compressed, or is it acceptable?

I know that brevity and conciseness are important, but I find it hard to fit all the information they want in a one page. And I have to list relevant experiences, future goals, research interests, and which people I would like to work with in the department.
Depending on the program I would keep it between 1.5 to 2 pages.  I framed mine to answer the following questions:


Why am I applying to this school.
What are my research interests.
Previous academic accomplishments.
Previous professional accomplishments.
Personal story of triumph (optional, I talk about having to drop out of high school to support my family)
What is your goal after graduation.


Addition:

I would also target schools that are pumping out the research that matches with your interests... this will help when you are stating why you are applying like:


  A major factor in helping me choose Awesome University was Professor Awesome's research on being awesome.  I feel that this aligns well with my research interests, and would make for a great fit between me and the department.


Also, if you talk about any abstract ideas be sure to elaborate on them to give a level of specificity.  They want to see you be able to translate abstract ideas into specific thoughts.