Interest in personal mobility has increased with the growing significance of first-mile and last-mile connectivity in smart mobility services. This study aims to propose a methodology for setting the service area of free-floating personal mobility sharing services and determining the optimal fleet size for the selected shared service area to address first-mile and last-mile challenges. We utilize population data, smart card data, and building data. Additionally, we estimate latent demand by incorporating age-specific and distance-specific utilization rates based on personal mobility device data. Along with the latent demand, we determine the service area based on locations of transit stops and buildings. We apply the proposed methodology to Yeongjong Island, Incheon. As a result, dense residential areas and popular beachside locations are designated as personal mobility sharing service areas. The fleet size for personal mobility in the dense residential service area is determined to be 1,022 units, while the fleet size for the beachside service area is set at 269 units.