I think that we should ask what people want to do with "characters" instead:
- If you want an array of very short strings, use an array. Do not abuse strings. Recent languages even don't like it.
- If you want a string that is not too long when displayed, first and foremost try to resolve that on the actual display (say, with CSS). If that's really impossible, pick a font and actually measure. Make sure that you are using the actual size being printed---the bounding box is not linearly scaled when the font size changes.
- If that display is a terminal, you may also consider using the East Asian Width [3]. But keep in mind that the default width of "Ambiguous" characters greatly varies across locales (Asians tend to prefer dual-width ambiguous characters, for example).
- If you want a string that is not too long when stored, use the encoded byte size. Oh, do you want to make sure that certain languages are not disadvantaged? Do your research to determine the appropriate limit per language then, but I bet you would be much better with generous limits (cough cough Twitter cough).
- (While this is not counting characters, for the completeness,) if you want to navigate a string with a cursor or a keyboard, then default grapheme clusters may actually fit a bill, as that might be the best possible! Someone will obviously complain though.
- If you have to, uh, really, really count characters, the chance is that you can assume a particular script and/or language. Reject others and use the local convention. You may want to read about the Unicode Script Property [4].
[1] https://unicode.org/reports/tr29/
[2] https://en.wikipedia.org/wiki/IJ_(digraph)