The Qubba of Imam Husain (AS) is the tomb above the holy shrine, where the Du’a is accepted according authentic Hadeeths in Sunni and Shia books الدعاء تحت قُبته.
( Kifayatul Athar by Al-Khazzar , page 9).
الحائر الحسيني Al-Haa’ir Al-Husaini is the area surrounding the holy shrine in which the traveler is allowed to pray his Prayers of full like at home. This is one of four places where the traveler is allowed to pray full ( 1. Masjid Al-Haraam in Makkah, 2. al-Masjid Al-Nabawi in Madina, 3. Al-Haa’ir Al-Husaini in Karbala and 4. Masjid Al-Kufa in Kufa, Iraq).
The circumference of Al-Haa’ir Al-Husaini is about 11.5 meters from all sides of the Holy Shrine. This is according to most authentic Hadeeths and verdicts of leading Ulama. There are other opinions regarding the circumference of Al-Haa’ir Al-Husaini which make it much wider, up to Kilometres, but such opinions are based on two weak narrations.