There's no way to turn off this feature, but I can assure you the
whole message is indexed and can be searched, so you don't have to
worry about that. I believe the clipping is to safe time putting the
message on your screen, as GMail is designed to be quick, and messages
beyond a certain length might take too long to bring up making people
think GMail is slow.
--
Sir Winston Churchill - "The reserve of modern assertions is
sometimes pushed to extremes, in which the fear of being...