Trace fossils of the Cambrian Gromia sphaerica were recently discovered off the coast of the Bahamas. Although paleontologists had previously assumed that these were multicellular organisms, this particular set of fossils indicate that these giants -- up to 3 cm in diameter -- were single-celled.
Link via Radley Balko
Link via Radley Balko
So what would the inside of a creature like that even look like?
Otherwise, I'm not sure that I've heard of a viable single celled organism that's that large.