The size of a mango depends on the variety; it is not an indicator of quality or ripeness. A perfectly ripe mango will have an intense, flowery fragrance and will yield slightly when touched. It should not smell fermented or have overtones of turpentine.
For the most common types available, the skin should show a blush of either yellow-orange or red, which will increase in area as the fruit ripens. (A Keitt mango may remain totally green with just the slightest trace of yellow even when fully ripe, so check for softness and fragrance instead of color.) A completely greenish-gray skin indicates a mango that will not ripen properly. Black speckles on the skin are characteristic of this fruit as it ripens, but an overabundance of black spots on a ripe mango may indicate damage to the flesh beneath. A loose or shriveled skin is also a sign of a mango past its prime.