As I know, hanging a picture (of a living thing) is not right. But recently I heard that it will be wrong only then when the size of the picture is identical with the original size of that living thing. Is there really any matter of size? If I hang a picture of a living thing in my room, as I know, angel can't enter in my room but devil can. Now at the same time, if I hang a picture of a mosque, suppose a picture of Prophets Mosque or calligraphy of suppose Ayat-ul-Kursi, at this situation can devil enter in my room? As I know, taking picture of living thing is not right. But I saw many pictures of mosques that have picture of human being as well. What about that? My last question is, Girls decorate their room with dolls, is it bad? As I know having a doll is Ok. Will you please discuss my questions? Thank you.

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The size of the picture does not change the rule concerning picture. Making pictures of living beings is forbidden regardless of their size. For more details on this subject please refer to the Fatawa: 83249, 83020, 82806 and 81999.
There is no evidence in Shariah proving that hanging the Picture of Kabah, mosque of the Prophet, or calligraphy of some verses of Qur'an, etc. keeps the Satans away from such a place. In fact, the Satans flee from the house in which the Surah (chapter of) al-Baqarah is recited. [Muslim] Ibn Hibban narrates that the Prophet said: "If the Surah-al-Baqarah is recited in a night the Satan will not enter that house for three nights and if that Surah is recited during a day time the Satan will not enter that house for three days". Some scholars put forward the reason that Surah-al-Baqarah contains many Names of Allah and His Rules so, the Satan flees from the recitation of this Surah.
Hanging the pictures of mosques, which contain pictures of living beings, is not permissible. Therefore, whoever intends to hang the pictures of any mosque should choose only the picture, which does not contain living beings in it.
The rule concerning the dolls of children is different from this rule, which confirms general prohibition of pictures. Many scholars believe that dolls are exempted from the rule of pictures for the Hadith of Aisha (Allah May be pleased with her) that she said: "When the Apostle of Allah (peace be upon him) arrived after the expedition to Tabuk or Khaybar (the narrator is doubtful), the draught raised an end of a curtain which was hung in front of her store-room, revealing some dolls which belonged to her. He asked: What is this? She replied: My dolls. Among them he saw a horse with wings made of rags, and asked: What is this I see among them? She replied: A horse. He asked: What is this that it has on it? She replied: Two wings. He asked: A horse with two wings? She replied: Have you not heard that Solomon had horses with wings? She said: Thereupon the Apostle of Allah (peace be upon him) laughed so heartily that I could see his molar teeth."(Abu Dawood)
There is another Hadith from Aisha (Allah may be pleased with her) that stated that "she used to play with her dolls in the company of young girls. Sometimes at that moment when the Prophet enters the girls go out and when he goes out they enter". [Abu Dawood]
Imam Ibn Hajar said: 'Scholars basing on this Hadith permitted dolls for young girls to play with. So, they exempted the rule of dolls from the general rule of prohibition of pictures'.
Allah knows best.