Amsterdam isn't the biggest city in the world - it doesn't have the slightly intimidating size of Tokyo or London, for instance. It's still a big city though, with a population (depending on how you calculate these things) of well over a million inhabitants. This means that it's big enough to feel like a major European capital, but not so big as to make tourists feel dwarfed by it. In fact, when you visit Amsterdam it's surprising how welcoming it is for a Capital city.
Stuff that has to be done when you go to Amsterdam:
Boat trip: you haven't really seen Amsterdam until you've seen it from the waterways. It really gives a new perspective on the city, and it's a great way to take in all the architecture at a good pace.
Van Gogh Museum - this place offers the kind of gallery experience you don't get very often. You'll probably have to queue to get in if you go at peak times, but it's an incredible place, and you'll leave with your knowledge of the artist's work and of The Netherlands itself much greater.
De Oude Kerk and Old Town - in the Old Town of Amsterdam lies the city's oldest building, de Oude Kerk (the Old Church), one of the main historical sights in the city's central areas. The rest of De Wallen is architecturally amazing, with tall buildings crowded along narrow streets, and tourists from all over the world checking out some of the wackier shop fronts.
Anne Frank House - walk around the house where the world famous Diary of Anne Frank was written. A brilliantly put together museum, and a must visit for anyone in Amsterdam.
The other great thing about Amsterdam is that if you don't speak Dutch, the people you meet are more than happy to speak English, so if you are from the UK, there is no language barrier.