If she is praying at home or anywhere, where there is No non-Mahram seeing her, or able to see her, then she does not need to cover anything below the ankles, but if she is praying where there is a possibility of non Mahram to see her, then it becomes compulsory on her to cover her ankles and and the whole foot.
We have got many authentic Hadeeths in this regard. ( Mustamsak al-‘Orwah al-Wothqa and Jawahiri al-Kalamazoo and other books).
It seems that in the time of the Prophet, wearing long clothing was a sign of showing off or pridefulness by men; therefore, some books indicate that the Prophet tended to tuck up his clothing. However the style of dressing was different then as compared to many places, and also the situation is not the same today.
However this is why in some interpretations of Sunni fiqh it is discouraged or condemned for men to wear trousers longer than the ankles. This is particularly common among Salafis.
But... do keep in mind... if your trousers are too long, you might trip!