A method performed by an object size mapping system is described for enabling improved positioning of a vehicle. The object size mapping system includes a first reference camera adapted to be arranged on-board a reference vehicle. The object size mapping system determines a current reference position of the reference vehicle. The system captures by the first reference camera, at the current reference position of the reference vehicle, a current reference image of a stationary physical reference object situated in the surroundings of the reference vehicle. The object size mapping system then determines a current reference size value of at least a portion of the stationary physical reference object, in the current reference image. The object size mapping system stores the current reference size value to be associated with the current reference position of the reference vehicle and a mapped digital reference object corresponding to the stationary physical reference object.