They vary in height. The largest is the Beijing Wheel, which was supposed to open in time for the Olympic Games this year - I presume it did?
I found a great site with lots of pictures, that lists 10 of the largest of them.
Beijing's 208m-high Ferris wheel is more than 50% taller than the 135m-high London Eye, and designed by the same people. The 198m-diameter Beijing wheel will rotate three times an hour, with each of its 48 capsules carrying 40 passengers. Like the London attraction, the 2,800-tonne wheel will be turned by 32 tyres running along its rim, powered by an almost maintenance-free hydraulic power system. Unlike the London wheel, passengers will be able to board from both sides of the Beijing attraction. They will climb aboard from platforms moving at the same speed as the turning wheel. The company building the Beijing Wheel, the Great Wheel Corporation, is looking at around 20 similar projects around the world.
But is the London Eye that I am most interested in, because that is where the book is set.
capsules: 32 carrying 25 passengers each
rotation speed: about 30 minutes/revolution
maximum capacity: 1600 passengers/hour
location: London, Great Britain
opening: December 31, 1999