What is the size limit in namecoin transaction? 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.
520 bytes for name_update to work

http://dot-bit.org/forum/viewtopic.php?f=2&amp;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.