Drosera magna is a rosetted tuberous species in the D. erythrorhiza complex. It is endemic to the sandplains between Perth and Geraldton. As its specific epithet suggests, the plants are particularly large in diameter, with mature plants typically reaching 10-12 cm across. This size separates it from Drosera erythrorhiza. It usually grows 4-5 active leaves, which are round and tapered facing the centre of the rosette (broadly obovate, almost flabellate). The shape of the leaf distinguishes it from D. squamosa and D. collina (which have elongated leaves).
Unlike Drosera erythrorhiza, which reproduces prolifically with stolons, D. magna only produces relatively few side stolons. These reach around 20 cm in length, resulting in colonies that are relatively sparse (in comparison to the very dense colonies of D. erythrorhiza).