reg:lseek&read ..pls

hi......

I stored few files into the database and trying to open it while accessing webserver...
In that case i need to just read the contents of the large object.
So...After importing all the files into the database i am trying to read the contents of the object.
But before that i need to know the size of the object from which i am trying to get the contents.
How can i do this?

void
pickout(PGconn *conn, Oid lobjId, int start, int len)

I used the above sample one which is in postgres document.
Here i am mentioning the Oid and start as 0 and len as 2000.....But i am unable to get the whole contents of the file.
If i want to get entire file contents...How to do?
What len i need to specify?
Is there any thing to know the size of the object?
Please tell me..............

Thank you
-Sandhya