The metatarsus of the heterodontosaurid MPEF-PV 3826 is one of the smallest known for Ornithischia, and size estimation based on this specimen may reveal new frontiers regarding the smallest sizes reached in the lineage. Aiming to estimate the body size of MPEF-PV 3826 and present a conservative model for size estimation in fragmentary fossil specimens, several linear regressions were performed relating the length of hind limb bones with body length and sampling different species along all major ornithischian lineages. For each case, the variables were analysed using linear regressions of raw and logarithm-transformed datasets, excluding the obtained outliers and considering possible gait-related partitions. The best models were chosen based on their adjustment, their predictive capabilities, locomotor morphotypes and the possibility of hind limb preservation. The best model selected to estimate the body length of MPEF-PV 3826, based on metatarsal-III length, revealed a 430.68 mm long body, which is amongst the smallest known for Heterodontosauridae. Linear models can represent a useful tool for body length estimation based on hind limb measurements of fragmentary ornithischian specimens. MPEF-PV 3826, an active component of the paleocommunity rather than a hatchling individual, is hereby proposed to represent one of the smallest ornithischian specimens ever known, playing an important role in the understanding of the paleoecology of Early Jurassic faunas.