Output size refers to the output size of specific cryptographic functions. It is especially relevant when the output must fit into a resource constrained system or protocol. For signatures it is the signature size, for KEM schemes it is the size of the encapsulated key.

Beware that there may be multiple ways of encoding the output. Sometimes the output is a simple binary string. For instance, the output of hash functions is well defined. However, for other algorithms the situation is not as clear. For instance, the output of ECDSA is a set (r, s), which may be encoded using constant sized r and s, or as a DER encoding of the same. Please make clear what is meant when asking questions about the output size of cryptographic algorithms and schemes.