Text

Text data type is used to store quite large textual data, for example, the body field of a node. MySQL does not allow TEXT data types to have a default value other than NULL. 
 

Sample Code:

$field['fieldname'] = array(
  'type' => 'text', 
  'size' => 'small', // tiny | small | normal | medium | big
  'not null' => FALSE, 
  'description' => t('stores fieldname for tablename table.'),
);

MySQL Field Mappings in schema.inc file :

'text:tiny'       => 'TINYTEXT',
'text:small'      => 'TINYTEXT',
'text:medium'     => 'MEDIUMTEXT',
'text:big'        => 'LONGTEXT',
'text:normal'     => 'TEXT',

Maximum Length Allowed:

Words Estimate:

Notes regarding words estimate:

• Words estimates are given here to help you select a text size for your field quickly.
• English words are calculated assuming an average of 4.8 bytes per word + 1 byte for interword spaces.
• Languages requiring multi-byte characters such as Greek, Arabic, Hebrew, Hindi, Thai, etc, etc typically require two bytes per character in UTF-8. We have assumed 11 bytes per word for these languages.

Storage Information :

Text is a variable-length data type. The required disk space depends only on the length of the stored string and not on the maximum length.

They take following storage space:

tiny /small  : L + 1 bytes, where L < 2^8
normal       : L + 2 bytes, where L < 2^16
medium       : L + 3 bytes, where L < 2^24
long         : L + 4 bytes, where L < 2^32

Note: L represents the actual length in bytes of a given string stored.
 

Similarities with varchar :

• Both are variable-length data types, the required disk space depends only on the length of the stored string and not on the maximum length.
   
• The maximum length of both varchar and text is 65,535 bytes.
   

Comparison with varchar :

• We can limit the max size of varchar fields but not of text.
• We can index varchar but the text is available for an only full-text index.
• If you need to have a default value that is not null, you have to use varchar.
• If you need storage space more than 64kB, you have to use medium/long text.
• Because of  storage implications, it is preferable to use varchar instead of text