What is the permissible length for trousers, and is it permissible for them to cover the ankles or must they be above them, knowing that what is below the ankles is specifically subject to the warning?

There is no harm in a Muslim covering his ankles. The prohibition and warning regarding Isbal (letting one's garment hang below the ankles) concern what falls below the ankles, as the Prophet, peace and blessings be upon him, said: "Whatever is below the ankles of the Izar (lower garment) is in the Fire."

The prohibition of Isbal applies to everything a man wears, whether trousers, a shirt, or anything else. The prohibition is not limited to the Izar or shirt.