While working with iTextSharp to fill AcroFields in a PDF, I noticed the font size of the text fields were a little larger than I wanted.
I found this great post that helped me out http://www.keithnordstrom.com/?tag=/itextsharp
which boils down to this:
AcroFields fields = stamper.AcroFields;
bool set = fields.SetFieldProperty(fieldName, "textsize", 12.0f, null);
The only thing to note is that you must set the form field size before putting text into the field otherwise it won't work...