title: Get the columns length of a data.frame
body: <p>I have a data.frame composed by N columns. The columns have not the same length (i.e. complete.cases), e.g. Column1: 314 rows, Column2: 543 rows, Column3: 124 rows, ...
  I would like to retrieve the length for each Column for the entire data.frame. 
  Can anyone help me please? </p>