Fix part of #13195 (Size not enforced)

[rluvaton]

Fix part of #13195 (Size not enforced)

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[rluvaton]

@googlebot I signed it!

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CLAs look good, thanks!

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[crisbeto]

Can you also change the commit message to something like docs(form-field): enforce input length in custom form field example?

[crisbeto]

LGTM

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