Differential coordination of growth and patterning across metazoans gives rise to a diversity of sizes and shapes at tissue, organ and organismal levels. Although tissue size and tissue function can be interdependent1,2,3,4,5, mechanisms that coordinate size and function remain poorly understood. Planarians are regenerative flatworms that bidirectionally scale their adult body size6,7 and reproduce asexually, via transverse fission, in a size-dependent manner8,9,10. This model offers a robust context to address the gap in knowledge that underlies the link between size and function. Here, by generating an optimized planarian fission protocol in Schmidtea mediterranea, we show that progeny number and the frequency of fission initiation are correlated with parent size. Fission progeny size is fixed by previously unidentified mechanically vulnerable planes spaced at an absolute distance along the anterior–posterior axis. An RNA interference screen of genes for anterior–posterior patterning uncovered components of the TGFβ and Wnt signalling pathways as regulators of the frequency of fission initiation rather than the position of fission planes. Finally, inhibition of Wnt and TGFβ signalling during growth altered the patterning of mechanosensory neurons—a neural subpopulation that is distributed in accordance with worm size and modulates fission behaviour. Our study identifies a role for TGFβ and Wnt in regulating size-dependent behaviour, and uncovers an interdependence between patterning, growth and neurological function.