This paper reviews the issue of population size (scale effects) in idea-based growth models. It addresses both weak and strong scale effects and incorporates the related distinctive features of the three strata of idea-based growth models. The paper also comments on third-generation models, emphasising their fragile framework due to the limited range of R&D spillover space they can accommodate. It is argued that because of the shortcomings of the third-generation models, a precise mapping of the relationship between population size and economic growth requires further research.