The rule of thumb in HVAC design is to size the system to meet the peak cooling and heating loads of the space. This means that the system should be able to provide enough cooling and heating capacity to meet the highest temperature and humidity levels that the space will experience. Additionally, the system should be designed to provide adequate air circulation and ventilation to ensure that the space is comfortable and healthy. Properly sizing the system is essential to ensure that the system is efficient and cost-effective.