This study initiated research aligned with the body positivity movement, aiming to explore size diversity for groups facing relative size discrimination due to their deviation from average body types. Using KS adult women's apparel dimensions as a reference, jackets were developed for women in their 20s to 30s who belong to the small petite-size (S[P]) category, which is characterized by a height under 155cm (petite) and a bust-circumference from 72cm to less than 82cm (small). Using 3D virtual-fitting, we conducted experiment-pattern production and refinement and subsequent real-fitting evaluations by participants to objectively validate aesthetics and comfort. The study’s findings are as follows: First, utilizing a 3D virtual-fitting program by identifying ‘creases’ and ‘garment pressure points’ in the jacket appearance, experiment patterns were refined and real jackets were produced. This approach addressed challenges in recruiting participants with specific body types and allowed for efficient research in terms of cost and time. Second, through real-fitting evaluations, basic-fit and slim-fit jackets labeled as <79-88-150> were developed for the S(P) size. we presented ‘size spec’ and ‘ease allowance’ for jackets by waist fit. Both fits received positive evaluations with approximately 53.5cm sleeve length, and 11.7cm shoulder length. The ease allowances for the basic-fit jacket were approximately 9.2cm at the bust circumference, 12.8cm at the waist circumference, and 6cm at the hip circumference. Similarly, the slim-fit jacket exhibited ease allowances of about 4.8cm at the bust circumference, 4cm at the waist circumference, and 4cm at the hip circumference, receiving positive evaluations for aesthetics and comfort.