All perfect praise be to Allah The Lord of the Worlds and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is recommended for the Imam as well as the person praying alone to recite, in the first two Rak`as and after al-Fatihah, any amount of the Quran. However, it is better to recite a complete Surah, even if it was of short or medium size in order to avoid breaking the recitation in case of a long Surah.
There are many a Hadith from which the scholars have arrived at the conclusion that it is recommended to recite long Surahs-in Fajir and Duhr prayers-from Hizb "Al-Mofassal", which starts from Surah "Al-Hujuraat." In Asr and Isha`, it is recommended to recite short Surahs from Hizb"Al-Mofassal", such as "Al-Doha" and "Al-Lail." In Maghrib prayer, it is recommended to recite short Surahs, such as "Al-Ikhlaas" and al-Moawidatain. However, it is permissible to repeat these short Surahs if they are the only ones known by heart.
Al-Imam An-Nawawi said: "Al-Shafie and other fellow scholars said: "It is recommended for the Imam as well as the person praying alone to recite, in the first two Rak`as and after Al-Fatihah, any amount of the Quran. But, it is better to recite a complete Surah-even short-than some verses from a long one………………………"{Al-Majmou`, vol.3/pp.349}. And Allah Knows Best.