Abstract Bistability in spinal motoneurons supports tonic spike activity in the absence of excitatory drive. Earlier work in adult preparations suggested that smaller motoneurons innervating slow antigravity muscle fibers are more likely to generate bistability for postural maintenance. However, whether large motoneurons innervating fast-fatigable muscle fibers display bistability related to postural tone is still controversial. To address this, we examined the relationship between soma size and bistability in lumbar ventrolateral α-motoneurons of ChAT-GFP and Hb9-GFP mice across different developmental stages: neonatal (P2-P7), young (P7-P14) and mature (P21-P25). We found that as neuron size increases, the prevalence of bistability rises. Smaller α-motoneurons lack bistability, while larger fast α-motoneurons (MMP-9 + /Hb9 + ) with a soma area ≥ 400µm 2 exhibit significantly higher bistability. Ionic currents associated with bistability, including the persistent Nav1.6 current, thermosensitive Trpm5 Ca 2+ -activated Na + current and the slowly inactivating Kv1.2 current, also scale with cell size. Serotonin evokes full bistability in large motoneurons with partial bistable properties, but not in small motoneurons. Our study provides important insights into the neural mechanisms underlying bistability and how motoneuron size dictates this process. New and Noteworthy Bistability is not a common feature of all mouse spinal motoneurons. It is absent in small, slow motoneurons but present in most large, fast motoneurons. This difference results from differential expression of ionic currents that enable bistability, which are highly expressed in large motoneurons but small or absent in small motoneurons. These results support a possible role for fast motoneurons in maintenance of tonic posture in addition to their known roles in fast movements.