We introduce a mesoscopic model to predict the charge mobility of organic semiconductors characterized by a coexistence of crystalline and amorphous phases. First, we validate our scheme by reproducing the trends in charge mobility observed in thin films of poly(3-hexylthiophene) (P3HT) polymers. Next, we address the problem of predicting the morphologies that lead to the highest mobility. Our main finding is the identification of a region of the model's multidimensional parameter space, in which the charge mobility effectively depends on a single morphological feature: the average intercrystallite distance. This scaling behavior provides insight into the main physical mechanism limiting charge mobility in organic semiconductors. Our proposed framework can be adapted to study a wide class of polymeric systems and used to guide the manufacturing of new, high-performing organic semiconductor materials.