What's the preferred datatype of the text area in which the length is unknown? The written text area might be small or large. For instance, I am collecting forum comments and won't know the size of a specific comment (area). What datatype is preferred?

BLOB or CLOB. According to your description, I believe CLOB (Character Large Object). Here is a spot to look.

However, you need to know ahead of time that although CLOBs are relatively simple to cope with, it might be more difficult than you planned. VARCHAR2s are frequently acceptable, if by "unknown" you mean under 4000 bytes. And together is fairly easy and easy.