By regulation, the minimum occupant cell size is based on a 50th percentile male (not including helmet space). Thus, as a team we have a say over who will be able to fit inside our vehicle.

ASC Regulation tl;dr 

The occupant space by regs is defined by an 835 mm arc pivoting about a designated point. Nothing can be within this arc with the exception of the steering wheel, mirrors, and seats. When occupants are seated normally, with safety-belts and helmets on, no part of the occupant nor the full free range of motion of the occupant's head (with helmet) may extend beyond the occupant cell (defined as roll cage + structural chassis)

Regulation Occupant Cell Size

10.3.C Occupant Space

• 10.3.C.1 The occupant space for each occupant’s upper torso shall be defined by an arc defined with an 835 mm radius measured from the hip point as defined in Appendix B of the occupant and projects forward 45 degrees from vertical, 25 degrees rearwards and 7 degrees side-to-side from the centerline of the occupant (this minimum occupant space requirement is based on a 50-percentile male and does not allow for a helmet. Taller team members may need more occupant space).
  
• 10.3.C.2 The solar car structure, including the windshield must lie wholly outside the occupant space. The steering wheel, mirrors, seat backs, and head restraints may be inside the occupant space but must be designed to minimize the risk of injury to the occupant.
  
• 10.3.C.3 The driver’s head must be above and behind the driver’s feet. The seat must be appropriately constructed with a solid base and back rest.

10.3.A Occupant Cell
10.3.A.7 The Occupant Cell shall encompass the occupant in all directions. When occupants are seated normally, with safety-belts and helmets on, no part of any occupant, nor the full free range of motion of the occupant’s head (including helmet), may intersect with the convex hull of the occupant cell.