You can't determine by scriptings if a fields value is too large for the
fields visible area. There is simply no property in the field widget
available that will inform you when this happens. The only information
is the visible plus sign [+] in the fields corner. The available methods
(such as "limit to visible area") to prevent of too long data to being
entered are only working for inputs done by a user BUT NOT when done by
scriptings or data loadings by the PDF viewer.