* Staphylococcus epidermidis is a bacterium typically found on the skin. It's generally small, around 0.5-1.5 micrometers in diameter.

* Escherichia coli is a common bacterium in the gut. It's also relatively small, with a size range of 0.5-1.0 micrometers.

* Bacillus subtilis is a rod-shaped bacterium that can form long chains of cells. It's significantly larger than the other two, with an average size of 0.7-1.0 micrometers wide and 2-3 micrometers long.

Therefore, based on size, Bacillus subtilis is the largest organism out of the three listed.