How to auto expand text fields/text box in pdf form with using java-script

Here's an article that explains PDF page coordinates and how to apply them to annotations and form fields

PDF Page Coordinates (page size, field placement, etc.)

Auto-sizing is a much more complicated topic and fraught with issues, such as how does the expanding/contracting field interact with everything else on the page? Do you just have a big blank area that represents the maximum expansion? Not to mention the issue raised by Try67, which is about know how exactly how big the field needs to be to fit the text. You can in fact measure text, but this is an advanced topic.

You can change the size of fields using a script, but not based on the amount of text they contain, if that's what you mean.