The aim of this study was to provide ergonomic recommendations concerning passenger compartment dimensions of an innovative vehicle dedicated to mail delivery. To this aim, an ergonomic analysis of egress/ingress postures has been performed on a physical mock-up, which is considered as a reliable tool when investigating driving space. Six workers with different anthropometries participated to the experiment. The influence of three seat heights, two headlining heights and three headlining widths were tested while participants performed a simulated mail delivery task. Based on goniometers and video observations, a Rapid Entire Body Assessment (REBA) was conducted. Perceived discomfort was estimated with a Category Partitioning Scale (CP-50). Results showed that REBA scores were mainly at medium risk (5.18 ± 1.75). Discomfort scores were significantly influenced by seat height (χ2 (2) = 7.79, p = 0.02), especially for short participants when seat height was equal to 760 mm (Z = -2.21, p = 0.03). REBA scores and discomfort scores were significantly higher for the lowest headlining height and the highest headlining width. Outcomes of this study permitted to establish that: the seat height has to be adjustable (between 580 mm and 760 mm), the headlining height has to be fixed to 1360 mm, and the headlining width has to be comprised between 300 and 525 mm. The results from this research suggest that physical mock-up can provide a useful tool for defining suitable dimensions for the design of a future vehicle, taking into account anthropometric characteristics of the target population.