The fact that no two mobile devices have the same screen size is one of mobile's big advantages -- and one of its drawbacks. It's an advantage for the user, who's free to pick the device size that best complements his preferences and work habits, but it's a disadvantage for the app creator, who has to design apps to scale across any number of devices and screen sizes.
A Forrester research paper, "Improving Enterprise Mobility: Meeting Next-Generation Demands for Development, Delivery, and Engagement," created in conjunction with Moovweb (makers of tools to transform Web UIs into mobile apps), delves into this and other issues.