This study aims to classify digital customizing types and analyze the customizing processes, sizing systems, and design components of domestic women's suits customizing services on online platforms. Based on the analyses, the study proposes 3D digital suit designs using avatar-based virtual fitting. The research methods are as follows. First, the digital customizing concepts and types are analyzed through a theoretical review. Second, case studies of domestic and international online suit customizing platforms from January 2022 to January 2024 were conducted to examine the digital customizing processes, sizing systems, and design components. Third, based on the findings, a digital customized suit design process for Korean women and 3D digital fashion designs using avatar virtual fitting are developed. The results of this study are as follows. First, digital customizing types are categorized into three types: individualization through combinational composition, customization through selective adjustment, and creation through assembly modules. Second, the digital customizing process consists of four stages: style selection, fabric selection, option selection, and size selection. The sizing system follows a four-step smart sizing approach, involving input of basic body information (gender, height, weight, age), ready-to-wear size data, body shape characteristics (abdomen, bust, hips), resulting in recommended measurements (bust, waist, hips, sleeve length, and garment length). Design components are selected in the following order: fit style, closure style, lapel style, hem style, and pocket style. Third, a 3D digital design for the customized suit is proposed, demonstrating how 3D digital technology can enable rapid design visualization, allow real-time modification without physical sample production, and supports eco-friendly customization through avatar-based virtual fitting. This study suggests practical applications of digital technologies in customizing design and is expected to contribute to the advancement of the domestic online customizing fashion industry by promoting digital customizing services and 3D virtual fitting technologies.