We all know that eukaryotes are bigger than prokaryotes. On average. Mostly. Of course our pathetic attempts at generalisation are too often devastated in a counterattack by nature’s awesomest power: variation. There’s variation within species, making it a necessity to ultimately tie biology back to populations from time to time — but that’s a topic for a later time. There’s even more variation in higher hierarchical orders, making it rather difficult to say much of anything about a group of organisms, sometimes. But the divide between bacteria and eukaryotes is surely a case where no ambiguity is cast, especially in something as obvious as size!
Of course, we later learn about very large bacteria, and very small protists, and the non-insignificant overlap between the two (this also happens between animals and protists — Polilov 2011 Arthropod Struc Dev). Ranging from 200-800 microns, Epulopiscium, a bacterial denizen of fish guts, is so massive, it was initially mistaken for a protist (Montgomery & Pollack 1988 J Eukaryotic Microbiol)! To give you a sense of scale, here’s a fairly famous image showing the giant next to Paramecium (also quite big for a protist, even) and E.coli reduced to tiny specks in the background.
