Jean,
We do pancreas most of the time. These are from hamsters and the whole
organ is still very small.  I would expect the rat pancreas to be about the
same size.  It fits in an ordinary cassete by just curving it a little.  It
is so thin and kind of brittle when processed so you may have to wipe the
block with a  dilute ammonia hydroxide on a kim wipe.
I imagine your rat pancreas would be similar to the hamster anyway.
Karen