How to change the font-size in PdfPTable?

Try this:

Font fontH1 = new Font(Currier, 16, Font.NORMAL);

PdfPTable table = new PdfPTable(1);

table.AddCell(new PdfPCell(new Phrase(yourDatabaseValue,fontH1)));

Please try by setting font to the PdfPTable.DefaultCell property

Example:

pdfTable.DefaultCell.Phrase = new Phrase() { Font = fontNormal };

I have already answer this before : Set font for all text from Pdfptable with Itextsharp