The instrumental resolution and the peak shape functions of the powder X-ray diffraction beamline BM16 at the ESRF is described with a basic set of parameters: the equatorial and axial sizes and divergences of the beam, the analyser and monochromator rocking widths. We show that the instrumental peak profile can be adequately fitted with a modified pseudo-Voigt function. With a nominal instrumental contribution to the peak breadth as low as 0.0025 degrees 2 theta, the measurable range of crystallite sizes can be extended into the micrometer domain. As an example the micro-structure of the NIST reference material SRM660 LaB6 has been evaluated.