Simulation of parallel and distributed applications is a very active research field as simulation allows for repeatable results, makes it possible to explore various platform scenarios at will, is not as labor-intensive or as costly as running experiments on a real platform, and often makes it possible to run enormous numbers of experiments quickly. If many simulation toolkits exist, most of them face the issue of the description of the large scale platforms that are currently deployed. In this technical report we focus on the platform description format used by the SimGrid toolkit and present how we modify its DTD to tackle the scaling issues induced by large platforms. Experiments show a reduction by a 6,600 factor of the size of the XML file for a multi-cluster platform comprising 1,300 hosts. We also extend the existing DTD in order to integrate the description of other features like attaching arbitrary properties to resources or introducing randomness in the platform description and express the dynamic nature of a platform directly in the XML file.