Yeah, just about a page, five paragraphs or so, mentioning your achievements (barring those listed already in the PS, though I guess it would look more stylish if someone else said how great you are as opposed to doing it yourself), your personality, your enthusiasm and your academic record. Some are insanely dramatic and others are fairly sober, it depends on the institution and the person writing it. Typical comments would be 'So and so has demonstrated great improvement/enthusiasm...', 'a joy to teach', 'highly motivated', 'x out of y pupils', 'top 5 in UK at...'. I've heard schools with a reputation for honesty have theirs taken more seriously than others which profess to be a genius factory, but it wouldn't usually be the most important aspect of the application. Few people will be able to tell you much though, as most schools don't let you see it.