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tokens: is inconsistent with a single Rayleigh distribution of inclinations ( i ). Its vertical distribution was better reproduced with two dynamical populations with typical inclination dispersions ( i rms ) of 1° and 9°. Such a scenario is analogous to the classical Kuiper belt, which also has a bimodal inclination distribution (e.g. Brown 2001 ). Similarly, ALMA observations of AU Mic marginally resolved its vertical extent (Daley et al. 2019 ) and constrained as well the distribution of inclinations to i rms ∼ 3°. For moderately inclined and narrow discs, the vertical extent can be constrained by comparing the surface brightness as a function of azimuth if the resolution is comparable or smaller than the disc width (Marino et al. 2016 ). This is because the non-zero vertical thickness of a disc widens the surface brightness distribution near the minor axis, hence reducing slightly its surface brightness compared to a flat disc. This principle has helped to marginally constrain the vertical thickness of HR 4796’s debris ring (Kennedy et al. 2018 ), finding i rms ∼ 3°. Similarly, it is also possible to determine the height of a wide disc if it contains narrow features such as a gap, as done by Marino et al. ( 2019 ) for HD 92945’s disc, finding i rms ∼ 4°. Note that the latter case is very similar to modelling done in protoplanetary discs with gaps to measure the degree of dust settling (e.g. Pinte et al. 2016 ). The width of debris discs provides as well a direct upper limit on the eccentricity levels. This is because for a uniform distribution of longitudes of pericentres, the difference between pericentre and apocentre distance must be equal or smaller than the disc width. This is particularly interesting when applied to narrow debris discs interacting with an eccentric planet since the final width of the disc strongly depends on the forced eccentricity induced by the planet (e.g. Pearce Wyatt 2014 ). Kennedy ( 2020 ) used these arguments to show that the narrowness of the debris disc around Fomalhaut (MacGregor et al. 2017 ) and HD 202628 (Faramaz et al. 2019 ) is inconsistent with the standard secular evolution theory. The dispersion of eccentricities in those discs, constrained by the disc widths, is smaller than expected under the presence of an eccentric planet. This inconsistency could be due to planetesimals being born on eccentric and aligned orbits or their relative velocities being damped reducing the proper eccentricities (e.g. through collisions; Nesvold et al. 2013 ). The multiwavelength analysis of disc sizes can also constrain stirring levels. By combining far-infrared (FIR) and mm spatial information with collisional evolution models, Geiler et al. ( 2019 ) found that the FIR emission of HR8799’s debris disc cannot be reproduced by a dynamically cold wide belt that best matches the mm data. They concluded that to fit both the large extension of the FIR emission and high mm flux, their model required the presence of two