Every trade portal has a maximum capacity of 25,000 items. That is NOT to say you can only send a maximum of 25,000 goods per portal. It rather means that the goods you send will be transported in batches of maximum 25,000 items. Transport time depends on the distance between the sender and the destination but also on the amount of goods sent (number of batches). However, transport time may also be subject to effects.
All the info on trade portals can be found in the Transports subadvisor screen, located under the Commerce Adviser.