This is quite a basic question.
Namecoin allows any arbitrary text as data:value pairs. How big can the data be? 100 bytes? 1 MB? I am not sure.
Bitcoin Stack Exchange is a question and answer site for Bitcoin crypto-currency enthusiasts. It only takes a minute to sign up.
Sign up to join this community520 bytes for name_update to work
http://dot-bit.org/forum/viewtopic.php?f=2&t=503
It should be twice that amount but there is some bug.
As you can read in the forum thread some planning is going on to increase it to 9kb.