The HTC Leo has a huge display! It's 800x480 and it reminds me of some of the older HP iPaq PDAs that had really large screens (of course, those devices were also quite big).
The HTC Leo has a 4.3" screen which is a huge jump from the old 2.8" screen. The HTC Touch Pro2 has a 3.6" screen (which is also the same screen size found on the Apple iPhone). As we near 5 inches, phones will start to resemble the old OQO model 02 (which had a 5" screen).