Worlds-in-Miniature (WiMs) are interactive worlds within a world and combine the advantages of an input space, a cartographicmap, and an overview+detail interface. They have been used across the extended virtuality spectrum for a variety of applications.Building on an analysis of examples of WiMs from the research literature we contribute a design space for WiMs based on sevendesign dimensions. Further, we expand upon existing definitions of WiMs to provide a definition that applies across the extendedreality spectrum. We identify the design dimensions of size-scope-scale, abstraction, geometry, reference frame, links, multiples, andvirtuality. Using our framework we describe existing Worlds-in-Miniature from the research literature and reveal unexplored researchareas. Finally, we generate new examples of WiMs using our framework to fill some of these gaps. With our findings, we identifyopportunities that can guide future research into WiMs.