Courier ships could range in size from small, one- or two-man ships like those that ferried Yoshi Raph-Elan and B'dard Tone to their destinations, all the way up to larger ships like the YKL-37R Nova and BR-23 that could transport troops and bulk cargo. In addition, courier ships were often well-armed to provide security for their important shipments.
There were also courier droids that could deliver messages or small cargo shipments from point to point or across the galaxy if need be.
Sentients worked as couriers in certain circumstances.[1][2][3]