Broad and narrow are certainly metaphors in most cases. They refer to measurements of certain kinds. They're not really the same as 'general' and 'specific', though they can be used that way in some contexts.
Literally, broad can be used to describe a river, a path, or a road (meaning 'wide'). Note that all of them have to do with unidirectional motion along some two-dimensional path, river, road, trail of footprints, etc. It is opposed by narrow (meaning 'thin') in these uses.
In non-literal metaphoric use, broad and narrow usually appear as part of a Path metaphor, like
Life is a Path
He takes the straight and narrow path to salvation.
Achieving a Goal is Following a Path
He gave that problem a broad clearance.
Thinking is Following a Path
We are in broad agreement about the plans.
In each of these the broadness or narrowness refers to the path involved, which is a further elaboration.
In the first one, narrow refers to the large amount of self-discipline required (or at least asserted) in order to stay within specified bounds.
In the second one, give a wide berth refers to avoiding a problem (itself a path metaphor) while moving to achieve some further goal -- wide refers specifically to the large distance between the moving agent and the problem avoided
In the third one, broad agreement means 'agreement in principle, without details specified' -- i.e, there is some distance between the two participants, which may get smaller as they move into closer (though not *narrower) agreement.
All of these and most metaphoric uses are idiomatic, and refer to larger metaphor schemas it's coherent with, like Path
or Container
.