size of a table on postgresql

Hello,

             
The following is the procedure to calculate the disk space occupied by
postgresql from a flat file.

In this I didn't understood some terms 

24 bytes: each row header (approximate)
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple

here row header is taken as 24 bytes and in some sites it is given
row header as 40 bytes and in some 32 bytes
http://www.sbras.ru/rus/docs/db/postgres/doc/howto/faq-english.shtml#3.6
http://www.softlab.ntua.gr/facilities/documentation/unix/postgres/faq-english.html

Is the row header value is constant or it is variable?If so what could be the maximun value?
what is that additional +4bytes as pointer on page to tuple.

Should all these were fixed or is there any command to find the values for these fields?

Please post your comments on it.

Thank You,
Avin.