[SQL Server] Determine or Predict the size of the Index for a table?

Hi,

Is there a way to determine the possible size the Indices of the table would take based on the table definition as well as the number of rows? The indexes are non-clustered, non-unique

If so, What about an index whose column will have null values?

Indices are basically tables that contain the actual column value and a pointer to that row.
So if it is a number field(bigint), the size should be roughly twice the number of rows * 8 bytes. etc.
Though I'm not sure about this.