Question:
What is the area in length and width that is to be shaved below the navel? Is it necessary to shave the hairs on the anus?
Answer: حامدا ومصليا ومسلما
A person will start shaving from directly below the navel.

يبتدئ في حلق العانة من تحت السرة

(الهندية 5:387)

The word العانة applies to both the hairs of the front and back private parts. Therefore it will be desirable to shave both. This has been explicitly mentioned by Allamah Zabidi in “Ithaaf Saadatil Muttaqeen”. (Extracted from Dhari aur Ambiyaa ki Sunnatain 50)

And Allah Ta’ala knows best.
(Hazrat Mufti) Ahmed Khanpuri (May Allah preserve him)
Mahoodul Fatawa 5/637
Also see here.