Forms page enforce the maximum number of characters allowed by Salesforce for a given field. For instance, if you include a Long Text Area field set to 5,000 maximum characters in Salesforce on a Forms page, this field will restrict the end user from submitting an entry of more than 5,000 characters.
There is a difference, however, between how Salesforce calculates the number of characters and how common applications like Microsoft Word or Google Docs calculate them. If an end user is seeing the error "[Field Name] must be less than XX characters" when attempting to submit a Form where their entries appear to include fewer characters than the maximum allowed in Salesforce, it may be due to this difference. See Accouting for differences in maximum character counts for Forms pages for further details and tips on addressing the difference.