How to use "of" after being verbesverbs

I found one sentence "Jockeys must be of diminutive size for their horses to compete":

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

whyWhy is there "of"?

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

Could you explain the usage of this "of"?

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asked Nov 28, 2017 at 22:39

How to use "of" after being verbes

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"?

grammar prepositions