Condottiere said:
I've seen mention of 300 and 2000 ton containers. Either the megacorps have their individual standardized freight, or more likely, the Imperium issues a set of guidelines for interstellar trade containers.
Keep in mind the maxim - form follows function. Containerized cargo traveling in the future will follows the same efficiencies that are followed today. Cargo being packed at the factor for delivery elsewhere (on-planet, in-system or extra-solar) will remain in the same container for as long as possible to get to the final delivery point. That's how most cargo moves in bulk today.
Cargo that doesn't move like that moves, usually, one of two other ways. The first is that packages destined for a location get loaded in bulk via containers/trucks and then are transported to a location where they get unloaded, resorted and reloaded for final destinations. This is a hub-and-spoke type form of transport. The final way is for outsized, specialized cargo's that require special transport or some other reason (for example outsized electronic drafting tables). These are packed in trucks and shipped from freight depot to freight depot, where they get mixed up in other cargo's to get to the closest depot destination, and are then loaded onto truck for final delivery. It's hub-and-spoke still, but far less efficient due to the type of the cargo. It's also more expensive.
Take for example physical mail packages. These would be collected from local points around a planetary system and collated into one shipment to the next system (assuming there isn't enough traffic to have them delivered directly to the final destination system). They would be loaded onto mail containers, sealed and then loaded onto bonded freighters (with gunz!). The freighter then jumps to the destination and unloads the containers, perhaps taking on another set and then moving again. The containers are taken to a central depot to go through customs and are then re-sorted for delivery to the planet/stations/planets in the system. At each point along the way they might get unloaded, re-sorted, then reloaded till they get to the local post offices for local delivery. What would work in the future for a package is no different than how a package is shipped across continents today, except you will substitute starships for aircraft, ships and trucks.
As GypsyComet pointed out, those large 300 and 2000 ton containers will most likely be used to transport multiple smaller containers, or bulk cargo (like grain or oversized cargo). It's faster to load/unload larger containers and then pick up new ones than it is to unload each container individually. That can be accomplished faster and cheaper and then your cargo gets pre-positioned for loading on the next scheduled transport. Today, at least in the US, there are rail cargo's called "hot-shots" that the major rail carriers schedule for cross-country trips. They are almost always high-priority cargo like mail, UPS/FedEx, or perishables. These trains only stop for fuel and crew-changes. If necessary, the engines get swapped out and the train never, ever gets broken up till it arrives at its destination. These trains are usually 10,000ft long (two US miles), carrying about 280 containers (usually 48 to 53ft each). That's somewhere in the neighborhood of 2,000 Dtons of freight.
Keep in mind that these containers will also need to get delivered to a location. So a giant 300 Dton container isn't exactly sized to be driven around town. Delivery vehicles will deliver 1-2, possibly as many as 4 semi-sized containers. Beyond that you probably will only see containers of that size being moved from shipping port to shipping port, except for special deliveries.