/qqq says "Instructions for the description the user creates to be one line only and between two to twelve words.", however, in general, Japanese sentences are not counted by words because words are not separated with spaces (instead counted by characters). The same thing could be said in Chinese, I guess.
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Would "two to twelve characters" be senseful?
Also @Shizhao I don't think this is suitable for adding it
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"The quick brown fox jumps over a lazy dog very very fast." roughly equals 30 characters, so I would say "equal or less than 30 characters". I estimate the exact number is not needed here.
Technically it should be less than 1000 chars, on current MediaWiki deployment.