The _length module is a protocol extension of PSYC and the routing layer MMP. It inserts length values into the PSYC syntax and allows variables to be transparent, that means to be of any length and to carry any information, even if that would normally conflict with the protocol syntax.
Independently from variable lengths there is also the body length negotiated by the _binary module.
Knowing length in advance is generally good, see Framing.
Most PSYC implementations support at least body length except for psyced which isn't binary transparent under all circumstances.
libpsyc implements variable lengths.