What is the default size of text field in Postgres? In the official documentation (http://www.postgresql.org/docs/9.1/static/datatype-character.html), it states text field is variable unlimited length.  

I wonder what is the default size of this field (or minimum size of the field)?
The default minimum size of a text column (representing a NULL value) depends on the rest of the row. In most cases it is absolutely free, if the NULL bitmap has room for another bit.


Does not using NULL in PostgreSQL still use a NULL bitmap in the header?


More about calculating the effective row size:


Configuring PostgreSQL for read performance