This description strongly implies that the ritual, once distinguished from the background magic of an area, has roughly the same visual footprint as a weather formation. The 'vanishing into the distance' bit, in this context, gives an impression of being cut off by the horizon. Further, the fact that it's 'inching towards' one *specific* person indicates that Mathilde can observe the path of the ritual from sky to ground quite precisely.
So yes, 'in a city of millions'. The description you gave made it seem like looking at a tornado and tracing where the funnel tip made landfall. If what you wanted to convey was that Mathilde's sight of the ritual-cloud was a) fuzzy and b) fairly tightly constrained in terms of horizontal distance, then what you achieved was the opposite.