The book defines a particular as a concept which does not allow shared application: it applies to one definite thing.
'Muʿayyan' means a specified individual. When 'Zayd' is used as a proper name to denote one identified man, another person is not included in that very referent.
'Zayd' is particular in that use. The reason is definiteness of reference, not the length of the word.
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The source definition of a particular
Universal and Particular — Definitions and Practical Classification
The book defines a particular as a concept which does not allow shared application: it applies to one definite thing.
'Muʿayyan' means a specified individual. When 'Zayd' is used as a proper name to denote one identified man, another person is not included in that very referent.
'Zayd' is particular in that use. The reason is definiteness of reference, not the length of the word.