This thesis explores two aspects of a decision support system for use during emergency situations: the design of a graphical user interface, and screen sizes. Through a usercentered design approach, these aspects are investigated based on the needs of what is referred to in this thesis as emergency leaders, which are the leaders responsible for the cooperation and coordination of an emergency response. Through evaluations of a prototype, in the form of a responsive web page, together with different sized screen, both design and device implications were derived. The research revealed five design implications which addresses the need for an Interface for use by emergency leaders to be role-based, context-aware, under the control of the user, scalable in relation to the magnitude of an ER, and to require limited input. The research also revealed that for the addressed users, a large tablet had the preferred screen size.