Need help in the design of a reefer container

Greetings of the day. Hoping every one is doing great.
Problem Description:
I am currently working on the design of a Reefer container to carry temperature sensitive things either below zero or above zero temperature. Am facing a problem in choosing the capacity of reefer unit i.e. if i want to maintain a temperature of -15 C then of which capacity reefer unit i have to select. Am working for many day but unable to that. I am looking for your guidance because am very new in this field. I shall be thank full for your response and support.