Always make a square symmetric data frame

I have to create a function that takes a data frame and makes it a square, symmetric data frame such that if the number of rows exceeds the number of columns, drop the first x number of rows such that the number of rows equals the number of columns (I have to do this vice-versa too).

For example, I need:

> df   
     a  b  c
  a  1  2  3 
  b  4  5  6

to look like:

> df   
     b  c
  a  2  3 
  b  5  6

Thanks.