These are the character limits applied to message templates:

Header

·    For media message templates, these cannot exceed 1024 characters, while standard templates are limited to 4096 characters.

·    When requesting the template the body is restricted to 1024 characters (parameters {{n}} count as 1 character).

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Body

·    For media message templates, these cannot exceed 1024 characters, while standard templates are limited to 4096 characters.

·    When requesting the template the body is restricted to 1024 characters (parameters {{n}} count as 1 character).

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Footer

·    Limited to 60 characters Buttons 

·    Caption is limited to 20 characters, while payload for the quick replies is 256 characters.