The text type handles tinytext, text, mediumtext, and longtext for MySQL, if that’s what you’re using. Just specify the upper bound using :limit => ...
Example:
change_column :articles, :body, :text, :limit => 4294967295
The default value of limit is 65535, as expected.
1 to 255 bytes: TINYTEXT
256 to 65535 bytes: TEXT
65536 to 16777215 bytes: MEDIUMTEXT
16777216 to 4294967295 bytes: LONGTEXT
The MySQL documentation can be found here.