In Bill Bryson’s A Short History of Nearly Everything, the author states the following (midway through chapter 2):
Now, Pluto has a diameter a little less than a fifth of Earth’s, so Bryson is saying in effect that a bacterium is a fifth of the size of a pea. That seems too large, but I don’t know enough biology to speak confidently on the upper bounds of bacterium size. Is this comparison wrong; if so, by what order of magnitude; if so, for extra credit, suggest an alternative for ‘bacterium’ in the quote above (‘size of a bogey/ booger’ seems about right, if lacking in scholarly elegance)
A fifth the diameter doesn’t mean a fifth the SIZE, as I am sure you know. It’s less than a hundredth the size of Earth, by volume.
It did seem like a weird comparison to me, too, though; you could certainly see Pluto on such a scale. Pluto would look about the size of a small crumb, which obviously you can see.
Bacteria come in many sizes, so he chose a poor comparison.
A Short History of Nearly Everything is an entertaining book, a good jumping-off point to help you figure out which science topics you’re interested in but know nothing about, and hopefully an inspiration to further reading. But please remember that some of the science in it ranges from A Bit Off to Complete Rubbish. Verify from an independent source. Bryson is wrong here, if entertainingly wrong.
The Thousand Yard Model of the Solar System recommends using a peppercorn - which is of course smaller than a pea - to represent the Earth, and says Pluto can be represented by a pinhead or a smaller object. A pinhead is really a bit too large for Pluto, but even at that scale Pluto would be visible, just very tiny.