It is easy for everyone, already from kindergarten age on, to judge and compare sizes and lengths. Which lollipop is biggest, that the Eiffel tower is tall, and that matchbox cars are smaller than real ones. But it is rather difficult to understand sizes at macroscopic and microscopic scale, because we never get to see it with the unaided eye, and most of us just see images taken by others.
I probably read hundreds and hundreds of times that a cell is around 20um; I vaguely remember that many bacteria are 1/10th of that size because one magnitude difference is easy to remember. But how much bigger a cell is than a virus, and how much smaller in relative terms than my finger, I read up on again and again.
To help myself, I started drawing the relative sizes of various biological entities that I am fascinated with. Myself (here: my thumb), a fruit fly (my model organism in research for 10 years), eggs of various sizes, cells and my beloved ribosome, a wonderful machine made of many proteins and importantly, RNA that exists in every organism.
While making the drawings and looking up sizes, I was once more mesmerized to re- discover that a membrane lipid is not that much bigger than a water molecule! And that a human egg, which itself is 10 times larger than an “average cell” is almost visible by eye! Also, consider this: cells come is vastly different sizes, the longest cell in the human body is around one meter long, while the smallest is around 10um. In other words, cells can vary in size over five magnitudes, from 10 to 1 000 000um! That means, if you think of the smallest cell as a tennis ball, the largest would be in comparison as tall as the Mount Everest (and, their nucleus is still the same size…)!
Have fun looking through the comparisons! A beautiful inspiration is here.
PS Also take note how one can use both relative size and scale bars for showing the size of an object! Please, never ever forget to add scale bars to your images, they are the only clue that allows your audience to relate the content to reality!
helena * jambor 
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