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Size and development time are universally important traits. Yet evolutionary trends in development time are often viewed as allometric and physiological by-products of initial size, when life-history theory predicts that both traits are targets of selection and evolve adaptively through opposing effects on fitness. Surprisingly, this prediction has rarely been tested by disentangling the direct and indirect effects of size and development time on fitness, as necessary to understand selection on each trait. Here, in a marine external fertiliser that provides novel scope for such tests, we measure directional, quadratic, and correlational selection acting on early size (of embryos and post-hatch larvae) and development time (from fertilisation to hatching) through survival of juveniles in the field. We find little directional selection acting on traits during this selective episode. Rather, selection is primarily correlational, targeting combinations of development time and post-development size in a way that acts against their already-weak positive correlation, and could eventually drive a negative correlation between them if persistent enough. Lack of correlational selection on combinations of embryo size and development time, in contrast, suggests that physiological or allometric constraints more likely explain their positive association. Hence, neither life-history theory nor principles of allometry and physiology alone may predict the evolution of size and development time, warranting greater appreciation of the tension between adaptive and non-adaptive explanations for evolutionary trends in these traits.