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Objective: People suffering from developmental dyscalculia (DD) are known to have impairment in numerical abilities and have been found to have weaker processing of countable magnitudes. However, not much research was done on their abilities to process noncountable magnitudes. An example of noncountable magnitude is conceptual size (e.g., mouse is small and elephant is big). Recently, we found that adults process conceptual size automatically. The current study examined automatic processing of conceptual size in students with DD and developmental dyslexia. Method: Conceptual and physical sizes were manipulated orthogonally to create congruent (e.g., a physically small apple compared to a physically large violin) and incongruent (e.g., a physically large apple compared to a physically small violin) conditions. Participants were presented with 2 objects and had to choose the larger one. Each trial began with an instruction to respond to the physical or to the conceptual dimension. Results: Control and the dyslexic groups presented automatic processing of both conceptual and physical sizes. The dyscalculic group presented automatic processing of physical size but not automaticity of processing conceptual size. Conclusion: Our results fit with previous findings of weaker magnitude representation in those with DD, specifically regarding noncountable magnitudes, and support theories of a shared neurocognitive substrate for different types of magnitudes.