This is the 7204th most frequent Russian word.
великим
"Great" or "grand" (instrumental case of "великий").
In this sentence, 'великим' is used as an instrumental singular of the adjective 'великий', meaning 'great', referring to someone distinguished or renowned in their field, here describing the writer Leo Tolstoy.
Лев Толстой был великим писателем.
Leo Tolstoy was a great writer.
Here, 'великим' is also the instrumental singular form of 'великий', emphasizing the significant or remarkable nature of someone's contribution to science.
Он прославился своим великим вкладом в науку.
He gained fame for his great contribution to science.
In this example, 'великим' is used to describe an aspirational characteristic (great leader), again being the instrumental singular to show the state or role associated with greatness.