How can I edit the size of the text field created in form controls?
You can't edit much with this application since it acts more like a VIEWER application rather than an EDITOR. You will need the professional version of it in order to open the file and edit its controls, including forms and other items.
You can either use the Nuance PowerPDF Converter: http://www.nuance.com/for-business/imaging-solutions/document-conversion/power-pdf-converter/advanced/index.htm
Or Adobe Acrobat Pro DC (recommended!): https://acrobat.adobe.com/us/en/acrobat/acrobat-pro.html