A method of operating a tile-based graphics processor that can use one of plural different rendering tile sizes is disclosed. The tile-based graphics processor includes a rasteriser that can rasterise primitives in a hierarchical manner. A patch size at which to begin the hierarchical testing of primitives in the rasteriser is selected based on the rendering tile size that is to be used. This can reduce the overall number of tests performed to rasterise primitives without impacting the correct functioning of the hierarchical rasterisation process.