Measurements of the aspect ratio of plagioclase grains in dolerites from sills. Photomicrographs (several per sample across each of the sills, at a magnification such that the long side of the field of view is 4.5mm long) are each accompanied by a drawing showing a pair of lines for each observable grain, giving the long and short axis of the grain intersection as viewed in thin section, together with a drawing showing only the long axes of each grain. A summary file provides the number of grains measured in each sample, together with the average aspect ratio, with 1σ uncertainties calculated using a bootstrap method. The grain size is reported for all grains measurable in the set of photomicrographs (in mm), and is taken as the length of the long axis for each grain intersection in thin section. The average grain size and the skew of the population is also provided. The choice of the area in each thin section to photograph and analyse was random, although any areas of late-stage alteration were avoided. This dataset is useful to anyone seeking to quantify plagioclase grain size in tabular mafic intrusions.