Even though standardisation of shipping containers has been in existence since 1967 its still not as specific as it may seem. Most people recognise 20ft and 40ft containers but in reality they vary between 10ft and 53ft in length (10ft are very common here in the Philippines for small loads). The heights also vary from 4.25ft to 9.5 so if buying a container getting the exact metrics internally and externally are important otherwise you could end up buying something unsuitable for your needs. The only thing that does seem to be standard is width at 8ft, what the containers are made from and internal dimensions vary between manufacturers and companies needs.
An example of this is Matson’s who after calculating the best sizes of container for their operations along the West coast of Hawaii set upon using 20ft and 24ft containers that fitted their converted freighters from WW2 and integrated a racking system into the freighters specifically for the containers. This allowed the containers to be stacked six high and to be easily offloaded at the dock side using specialist crane equipment.