Storing many small - and a few large - strings in an RDBMS
You failed to mention your DBMS but for Oracle and PostgreSQL you should simply put everything into a single CLOB column. 

The small values will automatically be stored "inline" and the large ones will be store out of line anyway. So the DBMS applies the logic of "switching" that you imply with your two column approach anyway. 

I would be surprised if other DBMS aren't doing the same thing.