Dial as per ASME B 40.1 is defined as the component that contains the scale and nomenclature. Dial size are varied depend on the accuracy required. More higher the accuracy usually requires bigger size of dial because of the division of scale will have more space thus more easily to read by a pointer.
Below is the minimum recommended size of dial as per ASME B 40.1 standard, while the project specification requirement usually bigger than this.