Assuming that your textbox is bound to a field in an underlying table, access the table in Design View and check the data type of that field. It should be set to 'Text' and the Field Size property should be set to 255.
It's possible that the data type may be set to 'Memo' which accepts 64000 characters.
If the textbox is unbound, then whenever you try to save it's value into a field then the string will be truncated to the Field Size of that underlying field.
Hope this helps
Grant