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Following the comments to this answer to another question, what is the difference between size and magnitude?

I know there's a difference, but can someone put it in a nutshell for me?

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What is it that you find confusing after reading the definitions? – KitFox♦ Aug 26 '11 at 11:28
I can't state it out in a logical way. I can't state their difference. Like what Matt Ellen said, he disagreed with what I said – Thursagen Aug 26 '11 at 11:30
I'm not sure the comments to the answer you're referencing use size and magnitude correctly, although they certainly serve to convey the intended meaning. – Peter Shor Aug 26 '11 at 13:08
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I don't think this one will fit in a nutshell. Anything you say about either definition can be contradicted by a counterexample or a counter-definition from a particular discipline (astronomy, mathematics, etc.) – JeffSahol Aug 26 '11 at 13:14
In science, magnitude also means the amount (or absolute value) of something without considering the direction. So a force of 10 N at an angle of 30° has a magnitude of 10 N. – mgb Aug 26 '11 at 16:53

5 Answers

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Usually, size connotes physical dimensions while magnitude connotes either a numerical measure (particularly a non-linear one) of any sort of amount or metaphorical size.

"What size is that screw?" (Physical dimensions)

"A problem of immense magnitude" (Metaphorical size)

You can also use size to mean metaphorical size, but you usually can't use magnitude alone to mean physical size. (Nobody would say "What's your sneaker magnitude?")

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+1: I think the suggestion that size is primarily used for physical dimensions while magnitude is more generally applicable sounds right. – AAT Aug 26 '11 at 22:35
I'm certainly going to be asking people for their "sneaker magnitude" after reading this. – gcbenison Apr 28 '12 at 3:33
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The difference/usage exists mainly in natural sciences (physics)

  • size: has nearly always a linear scale, if the size gets bigger, it also means higher positive absolute values of the according physical quantitiy (temperature, volume,...)
  • magnitude: has more the meaning "order of magnitude" and is often used esp. for logarithmic scales when linear scales are too unhandy and typical order of magnitudes like milli, nano, ... fail for practical use (e.g. magnitude of earthquakes, apparent magnitude in astronomy (here the unity is mag but a more negative mag value means a brighter astronomical object! so its counter intuitive to typical physical quantities.)).
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"Magnitude" connotes the quality of being big, whereas "size" is merely a measure of how big something is.

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Sounds very explanatory for me. – Tarik Aug 27 '11 at 23:38
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In mathematics, magnitude, together with direction, describes a vector. For instance, an object is said to have a velocity if it travels in a particular direction (say, "Northeast") at a particular rate (say, "30 knots"). The magnitude of velocity is speed, which does not imply a direction. Only values with a direction are said to have magnitude in this way; scalar values, such as an object's mass, or physical dimension (size) is never described in this way. Of course, there are many other ways that the word 'magnitude' is used

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Both have technical meanings - e.g. size in fashion, magnitude in cosmology. However, in general usage the difference is really a matter of register.

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