Highly inclined protoplanetary discs provide unique perspectives on dust evolution, allowing the vertical extent of the disc to be directly assessed. Further, polarised differential imaging (PDI) of highly inclined discs can probe the size distribution of grains in the disc's surface layers due to the size-dependence of the scattering phase function. I present our analysis of our multi-instrument observations of the highly inclined protoplanetary disc around Herbig Ae star HD 145718. By comparing synthetic images to our GPI PDI observations, we show that a reasonable assessment of the height of the scattering surface can be obtained by fitting ellipses to isophotes of $Q_{\phi}$ surface brightness. Moreover, we show this method has the potential to probe the degree of flaring in the scattering surface if the inclination is sufficiently well constrained. We use the TORUS radiative transfer code to examine the grain size distribution in the scattering surface. Our models are further constrained using an SED, constructed from archival photometry, and new and archival CHARA/MIRC-X, VLTI/GRAVITY and VLTI/PIONIER near-infrared interferometry. For typical ISM prescriptions of grain size number density, we find the largest grains in the scattering surface are ~0.5 microns in size. Larger grains have likely settled closer to the disc midplane. At the inner edge of the disc, the larger settled grains shelter small grains, allowing the sublimation rim to extend down to 0.17 au. The smaller grains emerge at larger scale heights beyond ~0.40 au.