I mean, correct me if I'm wrong, but IIRC cell size is mostly determined by the necessary rate of diffusion across the membrane and the surface area to volume ratio for a given size.

So, while there are some extreme outliers with more exotic cell biology, organisms having similar cellular metabolisms will generally have similarly sized cells, at least within an order of magnitude. Or in other words, an elephant is much larger than an ant because it has many more cells, not because its cells are much larger.

An exception to this of course being neural cells, which can be very very long, or very wide and branched (like Purkinje cells). But even within the brain this still kinda holds true. I actually know much more about brain anatomy than general biology, and I remember from the book Principles of Brain Evolution that elephant brains are much larger than ours, and actually have a much larger number of neurons, and that strangely intelligence seems to correlate more with the ratio between brain and body size than with absolute brain size. A possible explanation is that it may simply take a larger number of neurons to coordinate a larger number of muscle cells.

EDIT: case in point C. Elegans is about 1mm (or 1000 μm) long and has 1031 cells, including 302 neurons, which lines up with its cells being about as large as human cells when you consider that its a 3D volume and not a single chain of cells lined up next to each other.