Purpose: Leeway space is clinically crucial in pediatric dentistry because it is utilized to resolve tooth crowding and allow the first molars to drift mesially to establish a Class I molar relationship in the later stages of mixed dentition. This study investigated leeway space in the mixed dentition of Taiwanese children of different sexes and ages. Materials and Methods: The digital panoramic dental films of 182 lower arches of 119 boys and 63 girls aged 5–10 years were analyzed in this retrospective study. The mesiodistal crown widths of the primary canines and first and second molars and the permanent canines and first and second premolars were measured using medical imaging software. Differences in leeway space were statistically analyzed. Results: The average leeway space was 1.29 ± 1.48 mm on each side of the lower arch. The leeway space of children aged 5–6 years was significantly greater than that of children aged 7–8 years. No gender difference in crown width was discovered, except with regard to the primary first molar. Although no gender difference in leeway space was observed, permanent teeth affected leeway space more for girls than for boys. Conclusion: In Taiwanese children, although leeway space does not differ by sex, age affects leeway space. However, permanent tooth size has an influence on the leeway space of girls.