Igor's answer applies to US where you have 1-2 years to hang around a department before you choose a supervisor, and, maybe some other countries (Canada?). In majority of EU institutions, you will be given a Ph.D. supervisor straight when you come in. This makes your research statement crucial, as a professor will have to agree to take you on.
Igor's advice on writing two pages is applicable in EU too: nobody will read more than this. Make it a good narrative but avoid banalities from Wikipedia. Also remember that in EU, you are likely to attend an interview and if you cannot back your statement up by some background knowledge, it will look phoney.