India has a large variation in Sunlight exposure from region to region, due to its vast size. So amount of surplus vitamin D availity is subjective. Our body needs only 10 to 15 mins of bright sunlight light with UV ray exposure per day to fulfil vitamin D needs. Choose time so that your skin can bare sun intensity for 10 -15 mins. If you over expose yourself there are chances of skin burn .So choose time considering all these factor. By the way Concept of shadow size purely based on sun distance from objects. Nearer the sun larger the shadow and wise versa. So Dont go by it :)