Many programs actually specify a limit, which typically is two or three pages. But, even if exact limits are not specified, it is essential that your statement does not ramble about irrelevant things. Remember, your letter will be read by busy people — people who have many other applications to look at, and who will get annoyed if they have to spend more time than they want to reading any individual personal statement. A few short paragraphs covering one-and-a-half to two pages is almost always enough, unless the instructions in your application package specifies that you need to provide particular details that require more space than this.
Do not try to say everything you think might be relevant. Highlight two or three or maybe four important points and keep it at that. Before you start writing, plan the order in which you want to make your points. The people reading it will appreciate a concise and well-organized personal statement.