Hi welcome to H.C.M.Forum. there is no standard size for it, and it is not the time for you to think of it. any how you should be fit and healthy for it to be healthy. concentrate on studies, or on some work, extracuriculars. eat food rich in proteins
, and fats, fruits, green leafy vegetables to increase blood, drink more water, and juices. you will fulfil the norms.thank you.