If you want to buy a phone right now, and you're shopping based on quality rather than price, you have two choices in terms of size. You can get the iPhone, with its 3.5-inch screen, or you can choose from a handful of top-tier Android and Windows phones, all of which will have, at the bare minimum, a four-inch screen. Most of them will be bigger--4.3 inches is much more common right now, and an increasing number are even larger, including the Samsung Galaxy Nexus (4.65 in), HTC Titan (4.7 in), and the Samsung Galaxy Note (which, at 5.3 inches, is more lunchtray than phone).
The Nokia Lumia 900 is essentially a 4.3-inch version of the Lumia 800, a phone I absolutely loved in its 3.7-inch iteration (a Europe-only model). So reviewing the Lumia 900 presents an interesting question: with most other specs remaining constant, how does the experience of using a phone change when it grows to the size most phone manufacturers insist we really want?