Dear sbettal,
For the drag/drop functionality, it's already implemented in the demo project .. please check PDFLayoutView.onTouchAnnot (needs a professional or premium license).
For the listener to editbox annotation, it's implemented in the demo project in PDFLayoutView.onSingleTapConfirmed that calls PDFLayoutView.onEditAnnot (premium license).
To increase the editbox height, you can add a listener to check the entered text length and adapte the editbox accordingly, using Annotation.SetRect.
But as a warning it will not be optimal (performance wise)..as a modification on the annotation level, requires a re-render. (so try to do the re-render once after the user has finished entering the text)