The demand for extended-size men’s suits is growing due to the increasing population of overweight and obese men in the U.S. However, the big and tall demographic remains underserved with limited fit and design options. This research examines the use of 3D apparel design software, specifically CLO3D (version 2024.0), to develop 3D avatars representing diverse body shapes and sizes. By incorporating Body Mass Index (BMI) and body shape, the study aims to enhance fit for this demographic and facilitate the development of inclusive 3D avatars for virtual prototype fittings. Findings from this research reveal critical insights from online customer reviews and 3D avatar development, identifying common fit issues in areas such as the midsection, armhole, sleeve length, and pant dimensions. These issues emphasize the need for improvements in extended-size men’s suit designs due to the limitations of the current men’s sizing system. The study explores how realistic 3D avatars can minimize customer’s alteration needs, providing valuable insights for designers and product developers. By enhancing fit accuracy and promoting design diversity, this approach contributes to a more inclusive menswear market and lays the groundwork for future menswear innovations.