Grassland management impacts ecosystem functioning and is critical for livestock production and biodiversity conservation. Decreasing the stocking rate usually benefits biodiversity but also decreases production per unit area. Here, we assessed the combined effects of cattle herd and pasture size on sward structure, diet quality and daily grazing time while maintaining a constant stocking rate. The experiment was conducted over four successive years in two blocks of a species-rich upland grassland that was continuously grazed for 5 months between May and October. Each block consisted of one 3-ha pasture that was grazed by a group of three heifers, one 9-ha pasture grazed by nine heifers, and one 27-ha pasture grazed by 27 heifers. The grazed patches were discriminated using a mathematical model that for each point gave a probability of being grazed based on sward height and spatial dependence. Heifers created fewer patches and had less control over sward height in the smallest (3-ha) plots, while no difference was found between the 9- and 27-ha plots. The small heifer groups also had the shortest daily grazing time. These observations point out a risk that animal impacts on sward structure could be partly missed in grazing experiments conducted with very small groups. Estimated diet quality was unaffected by pasture size. Patch stability was frequent between two successive years but we observed a decrease in interannual patch stability as time intervals increase. Such observations are useful for calibrating spatial interaction models combining foraging behaviour and vegetation dynamics rules and enhance their predictive ability.