The objective of this study is the development of crown width–tree height (CW-H) models in cities in northeastern Greece, for better urban vegetation management. In the cities of Kavala, Xanthi, Komotini, Alexadroupolis, and Orestiada, in total, 700 trees of Magnolia grandiflora L., Prunus cerasifera Ehrh., and Acer negundo L. were selected. For each selected tree, the total height as well as the minimum and maximum crown diameter were measured, and the average of the minimum and maximum diameter was considered the crown width. The selected CW-H models exhibit satisfactory R2 values of 0.60 and above. There is not a common pattern in the value order (or rank) of R2 among the M. grandiflora, P. cerasifera, and A. negundo CW-H models in the studied cities. A medium-sized tree such as M. grandiflora, a small-medium sized tree such as A. negundo, and a shrub or small tree such as P. cerasifera did not exhibit substantial differences in estimated and observed crown widths. The results of this study will increase our understanding of CW-H allometries. The main use of these models will be in the selection of the proper above-ground spacing of tree lines and in the spatial planning of a tree’s establishment so that no problems arise from its proximity to infrastructure.