AbstractAn evolutionary approach to reproductive allocation is extended to the realm of size‐specific demography. Questions of how resource is allocated between male and female functions and between present and future reproduction are investigated. The concepts of reproductive value and the Euler‐Lotka equation are extended to cover size‐specific demography. The criterion of life history evolution in age‐specific demography is also extended and applied to the prediction of the reproductive allocation pattern selected for in accordance with the characteristics of a plant species. The theory is applied to the problem of sex change. Our predictions are: (1) Sex allocation pattern will change according to the size of the plant if the increase in reproductive success, growth or survival rate with increasing size is different between sexes. (2) Sex change (from one sex to the other) is selected for if the plant has a concave tradeoff relationship between male and female reproductive success in each reproductive season. Gradual sex change (relative increase of resource allocation to one sex as the plant increases in size) is selected for if the plant has a convex trade‐off relationship. Also by this extension of reproductive allocation theory to size‐specific demography, testability of the theory with field data is greatly increased because it enables us to examine the plants in which size rather than age is a better predictor of demographic