Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / L /

calculate the length of a vector with the components selected in the Nodes Properties

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/length.xhtml

•X

Specifies a vector of which to calculate the length.

Component Selection of the Vector can be performed in the nodes properties

•Output

Returns the Vector Length i.e.