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.
A potential source of confusion for end users, however, is the difference between how Salesforce calculates the number of characters and how common applications like Microsoft Word or Google Docs calculate them. For the former, any line break is counted as two characters. For the latter, it is counted as only one.
If an end user is using one of those common applications to count the number of characters of their submission prior to entering it into the form and attempts to use all available characters, their entry will be over the allowable character limit if they have included line breaks.
It is recommended that for any field you include on a Forms page that will allow end users to use line breaks, the stated character limit on the form should be less than the actual character limit in Salesforce to account for a reasonable number of line breaks the end user might include.
For instance, if you have a Long Text Area field set to 5,000 maximum characters that is asking for a Project Description from the end user in support of a grant application, you may wish to 1) set the label on the form to say "Project Description (4,750 maximum characters)"; or, conversely 2) increase the maximum characters of the Salesforce field to, say, 5,500 and set the label on the form to say "Project Description (5,000 maximum characters)".