In tablet mode the screen on the echo measures 4.7 inches.
The Evo screen is 4.3 inches.
There are pros and cons to both phones, the Evo does 4G, while the Echo does not. The Echo does some interesting things where two apps can run at the same time on the different screens.
And yes, there may be another phone on another network with a bigger screen. If you are on Sprint (which I assume you are) you might also consider the Nexus S, which runs faster and has a clean, simple design. Also, the Evo 3D is supposed to hit stores this Friday-- it will feature dual cameras to take 3D (HD) video.