An acknowledgement has an allowed length of 255 characters.

If it exceeds 255 characters, the acknowledge button wil raise an error returning:

ERROR: 'Cannot acknowledge event'
Value: "acknowledge input text" is too long for field "message" - 456 characters. Allowed length is 255 characters.

If you exceed 255 characters and hit the 'Acknowledge and return' button it won't raise an error and won't save the text either.