This interdisciplinary treatise integrates notions from the biological, geographical, sociological, politico-administrative, economic, psychological, futurist, and other scientific literature about the expansion of urban areas by taking the reader through a series of conjectures about the practical upper limits of the size of cities, and centering the discussion around the possibilities for a world-spanning megalopolis or city-planet. It specifically frames urban growth against a survey of known conceptual and logical limits established by previous research in the natural and social sciences, and demonstrates that while there are absolute and practical constraints to the establishment of what the author calls a Pangaean City, they might be overcome by new technologies, innovations in governance, and behavioral adjustments. The author also shows that prior, overlapping, or parallel sociopolitical and cultural constraints govern city size, and that while these are not immutable, they represent sets of actual influences on development of the magnified urban form itself, including its reach beyond physical presence. Finally, while it is shown that a thorough-going planetary urbanization may be physically impossible, the research concludes by suggesting what planners can or should do about such a phenomenon. It further relates the discussion to the rich body of utopian planning literature, where the aspiration for an all-embracing urbanity remains to mirror the panoramic analyses of this paper.