2-5 So he would recite the lengthy soorahs of al-mufassal such as Qaaf (50), “Alif-Laam-Meem tanzeel”, and al-Mulk (67); and he would recite Soorat al-Mu’minoon (23), as-Saaffaat (37) and so on…
And sometimes he would make it brief, either because he was travelling ...

3-5 or for some other reason, as he ﷺ said: “I may start to pray, intending to make it lengthy, then I hear the crying of a child so I make it brief because of what I know of his mother’s concern for him.”

4-5 And it was narrated from him that he used to recite in Fajr Soorat at-Takweer (81) and Soorat az-Zalzalah (99).
So the imam should usually do what the Prophet ﷺ usually used to do ...

5-5 ... then if there is a purpose to be served by making it longer than that or shorter than that, then he should do so, as the Prophet ﷺ sometimes used to do more than that or less than that.
(Majmoo’ al-Fataawa, 22/318)