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In this study, we examined density-size relationships for Sitka spruce (Picea sitchensis (Bong.) Carr.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) using data collected in stands in Great Britain (GB) and Western Canada. These two conifers are native to Western North America and have been widely planted in GB. Results indicate differences between stands in Canada and GB in both the intercept and slope of the log maximum density-log size boundary lines. In GB, the slope (b) of the relationship between log of stand density (number of trees per hectare) and log of quadratic mean diameter (D q) is steeper than the theoretical value of −1.605 (−2.063 for Sitka spruce and −1.864 for Douglas-fir). Values of b are lower in Canada (−1.437 for Sitka spruce and −1.241 for Douglas-fir) than in GB. Within each region, b is similar for the two species. However, the intercept term differs for Sitka spruce and Douglas-fir in GB. These differences provide additional evidence that density-size boundary line relationships are influenced by environmental and other factors and indicate the need for development of density-size relationships for each species and for each region where the species is grown. Maximum stand density index (SDI) values calculated using these relationships are 1868 and 2073 for Sitka spruce and 1491 and 1815 for Douglas-fir in GB and Canada, respectively. Differences in maximum SDI between these two regions may be related to differences in climate, provenance, stand history and other factors. Maximum density-size relationships presented in this paper can be used as a starting point for managing stand density for both even-aged and continuous cover stands and for identifying potential maximum stocking in stands of these species in GB and Canada.