there's no real limit (its size is described with 32bit number, and
that's the only limitation here).
But you need to be aware, that content is sent over at once, so memory
is the limit in your case.
http://www.postgresql.org/docs/8.3/static/datatype-binary.html
For such large objects, it might be actually better to store them
separate as files, and just store file name.
