How to use "of" after being verbs

I found one sentence:

Jockeys must be of diminutive size for their horses to compete.

Why is there "of"?

Is it wrong to say "must be diminutive size"?

Could you explain the usage of this "of"?

"Diminutive size" is a noun phrase.

While noun phrases can be the complement of "be", they always indicate the identity or nature of the subject, not a property associated with it: a person can be a teacher, a human, a child, a monster, a tyrant, your uncle, the King of France, or (figuratively) a dinosaur; but a person cannot be a size, diminutive or otherwise: sizes are fundamentally different kinds of things from people, in fact from any kind of physical object.

Adjectival and prepositional phrases, on the other hand, can express a property or quality of the subject as opposed to its identity. So the jockey can be diminutive, or of diminutive size, but he cannot be (the sme thing as the abstract concept) diminutive size.