The traditional universal sizing system, which emphasizes broad adaptability, struggles to precisely meet the body shape characteristics of target consumers in niche markets. This mismatch frequently leads to sizing inconsistencies for apparel companies, further causing overproduction and resource wastage. This study addresses the demands of segmented markets by integrating the concepts of precision and mass customization, proposing an innovative solution for a customized sizing system. Drawing on an anthropometric datasets comprising 600 Malaysian females, a two-tiered SOM-K-means clustering methodology was employed to develop a highly adaptable, customized sizing system. The study successfully developed a customized sizing system comprising 38 sizes for a Malaysian tailor brand, covering over 96.8% of the target population with an overall aggregate loss of only 1.93 cm. The results demonstrate the high precision of the customized sizing system, significantly improving size compatibility and product fit for the target market. This customized sizing system provides a reliable size reference for apparel companies in niche markets engaging in mass customization, effectively enhancing fit, product development efficiency, and reducing resource waste, thus advancing the apparel industry toward circular economy and sustainable development.