What is the type of the field you are hiding? I had some discussions with supoort on character limits in the past.
According to the engineering team, string field types have a limit of 256 across the board, as do numerical fields (with the exception of phone number which is 10). The text field type has a limit of 3200. You may want to consider changing the field type to text if you are looking to store more than 256 characters in the field.