All Purpose sign extension VHDL?

Hi, I was just wondering how it would be possible to make an all purpose 
sign extension entity in VHDL.


So I created this sign extension entity but it will only work if the 
input is 6 bits long. If I put in an input that was say 3 bits long. It 
wouldn't work. Is it possible to build a SEXT that doesn't have 
limitation on input bits?

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity SEXT5_0 is
  port ( data_in : in std_logic_vector(5 downto 0);
         data_out : out std_logic_vector(15 downto 0));
end SEXT5_0;


architecture behavioral of SEXT5_0 is
begin
  process (Input)
begin


  data_out(5 downto 0) <= data_in;
  data_out(15 downto 16) <= (15 downto 6 => data_in(5));

end process;
end behavioral;