In order to assess the experimental artefacts associated with image-based characterization of aggregates, tests were performed to evaluate the effects of aggregate size and shape, relative to the pixel resolution of the imaging device. The basic motivation for the study was to further the understanding of particle separation processes in water treatment and specific goals were to: (1) analyse flocs developed from coagulation and flocculation processes using a non-intrusive measurement technique; and (2) develop relationships between measured aggregate characteristics and accuracy in an imaging method. The main objective of this study was to investigate the dependence of perceived aggregate size on pixel resolution using an imaging method. Analysis of fractal dimensions as a means of quantifying aggregate shape and other geometric properties was used to illustrate the consequences of potential measurement errors that might be generated, based on a given pixel resolution. A variety of pixel resolutions might be realized in an experimental setting by using different camera lenses and magnifications, and this variety is related to the desired degree of measurement accuracy. A relationship between the expected measurement error and the particle properties is established as a function of pixel resolution for both a single spherical particle and a cluster of three spheres. Results of this study provide an estimate to quantify the error in measurements due to pixel resolution obtained from an imaging method depending on the size of the aggregates of interest, and to assess the appropriateness of a particular pixel resolution depending on the desired degree of accuracy for a given analysis.