Punctuation with units

I remember reading somewhere that if a unit is abbreviated as one character, there must not be a space between the number and the unit (e.g., 5m, 26K). If the unit is abbreviated as two or more Characters, there must be a space between the number and the unit (e.g., "10 km", "USD 5").

• Can you please help me find the source again?
• Is this recommendation correct?

If you're typesetting SI units, it seems logical to follow the conventions of the Bureau international des poids et mesures. From the SI Brochure, §5.3.3:

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. (…) The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle, °, ′, and ″, respectively, for which no space is left between the numerical value and the unit symbol.

§5.3.7 goes on to say that “When it is used, a space separates the number and the symbol %.”

In practice, it is quite common to see non-alphabetic units such as % and °C typeset without an intervening space. I've never seen a rule that distinguished between single-letter units and longer units.

Note that the rule doesn't specify how wide the space should be. Some references recommend a normal inter-word space, while others recommend a thin space. In any case, the space is nonbreakable.

These rules need not apply to currencies, especially when they are written before the number. Specifying that single-character currencies don't take a space ($42, £42, €42) while multiple-character currencies do (AUD 42, A$ 42) doesn't feel completely outlandish, maybe that's what you remember?

In Canadian Style: A Guide to Writing and Editing:

• When symbols are used, the prefix symbol and unit symbols are run
  together:

  5 cm
  7 hL
  4 dag
  13 kPa
  

• When a symbol consists entirely of letters, leave a full space between
  the quantity and the symbol:

  45 kg not 45kg
  

• When the symbol includes a non-letter character as well as letter, leave no space:

  32°C not 32° C or 32 °C
  

However, the International System of Units, or SI, requires a space to be used to separate the unit symbol from the numerical value, and this also applies to the symbol for the degree Celsius, as 32 °C. The only exceptions to this rule in the SI are for the symbols for degree, minute and second for plane angle, as 30° 22′ 8″. Wikipedia's style guide also follows the SI standard.

• For the sake of clarity, a hyphen may be inserted between a numeral and a symbol used adjectivally:

  35-mm film
  60-W bulb
  

However, some other style guides, including Wikipedia's, deprecate hyphenation in these cases. The SI allows a hyphen between the numeral and the unit only when the name of the unit is spelled out, as 35-millimetre film.

Source: https://en.wikipedia.org/wiki/Space_%28punctuation%29#Unit_symbols_and_numbers

Addressing the OP's second question and setting currencies aside, the answer may be that both are 'correct' as long as you are consistent.

This may be an example of where everyday usage is at variance with style guides. I'm sure that most academic papers will follow the advice of number + space + measurement, but if you do an image search for food labels for example, they seem to almost invariably omit the space between the number and the weight. E.g. '200g' and not '200 g', and 'Fat: 8%' rather than 'Fat: 8 %'. This may simply be due to space being at a premium on a label, but I suspect it's more widespread than that.

This is not to say that the style guides are 'wrong' but may suggest that it's now widely acceptable to omit the space and that perhaps the convention is changing over time. A quick search for '200g' vs '200 g' using Google's Ngram viewer suggests that the usage of measurement without the space may have caught up with or even overtaken 'number + space + g' over the last two decades.