Designing for different screens/devices needs some careful thought.

Different screen, different experience | Photo by Roberto Nickson

When creating your mockups and planning for your users you need to consider the size of the screen that your users are going to be creating for and design accordingly. Currently the best way to do this is to start at mobile and design up. This means that you should sketch out your initial wireframes and hi-fidelity mockups as they would appear on a mobile and then a tablet and finally a desktop.