I use 4 dton 3m x 3mx 6m as the basic size. "4 tons" of cargo is cargo that will fit (cube and/or weight) into this. 1 and 2 dton containers can be combined into this footprint. Also, 27 x 1 ml gross "cubes" can be contained, nicely to a 2dton footprint.
Some cargo will be "break-bulk," in transit. More labor, and more chance of damage.
Ballard Designs has designs of standard starships that are optimized for fitting 4dton modules. 30 dton cutter modules fit 7, and have a 2 dton accessway, which allows these to be stacked like Legos. This space can also be used to stack in break-bulk cargo at the risk of breaking various regulations on access.
A modular cutter can thus carry 1 module with 28+ dtons of cargo at normal performance, and remain streamlined. It can carry a total of 5, however, at a maximum of 1g, while losing streamlining. A Ballard type A (Type AB) carries two, and allows space for an additional 6 4 ton modules. The modules allow for 4 dton inserts that can reconfigure the interior space for middle passage and carying mercs.
Standard 4dton cargo containers cost kCr1 new, and are typically leased, (Cr50 per jump or Cr 100 per year) traded one for one, or sold as part of more pricey cargos, depending on the local nature of trade. They are airtight, and lightly insulated. Climate controlled containers are 5kCr, and require power hookups or refueling of H2. They are often the basis for housing and other structures in startowns. A class A or B starport will have a Ballard Leasing agency (or local equivalent) that will facilitate such leasing. Well used containers are available for Cr200, but will require ongoing monitoring and maintenance or risk damage to cargo.