Channel length (L), the most important parameter of various transistors, determines the transistor performance and integration density. Emerging vertical organic electrochemical transistors (vOECTs), easily enabling L of less than 100 nm, show extremely high transconductance (g_m) and small footprints. However, as a new device structure, systematic studies on the limit and effects of the L on transistor performances remain unexplored. Here, by adjusting the concentration of the organic mixed ionic-electronic conductor (OMIEC) channel solution, vOECTs with L from ∼100 nm to ∼15 nm are fabricated and characterized. Surprisingly, shorter L would not lead to higher transistor performance, where the highest peak g_m of ∼183 mS is obtained with ∼20 nm L. Through comparing the relationship between L, g_m, and response time, it was found that the influence of L on device performance decreases gradually when the L is reduced to below 50 nm. This is mainly due to the decreased ion injection capability resulting from the reduced channel thickness. This work demonstrates an optimal design of vOECTs channel geometry that will lead to state-of-the-art transistor performance and also provides insights into ionic-electronic coupling mechanisms in the nanoscale.