I am keen to find serious estimates of the size of the markets for
consulting services provided to the corporate sector helping clients
(a) assess the skills and capabilities of their existing mid-level and
senior managers and (b) provide outside executive coaches to such
managers. Such assessment is often (but not always) based upon
psychpmetric testing but is also increasingly based upon structured
interviewing.

A great answer will point me towards several estimates, with a
preference for those who explain the assumptions behind the estimate.
I am aware that there are important definitional issues involved here
and that estimates are likely to vary considerably.