20 point is too small to make out the unit sizes marked above the icon. Plus, the finer details in 20 are difficult to discern clearly. I can tell what the icon is, but if you're dealing with someone who doesn't know the icons readily, they'd have a problem.
28 point is probably the absolute smallest you'd want to go for the everyday person who's going to have various settings on their computer for screen resolution and various levels of eyesight effectiveness.
My personal recommendation would be 32 point. It won't take up as much space as the larger size and isn't quite an eye strain for some people who might have different settings or different eyesight.
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