Click on the text-tool in the toolbar, now double click on the text you want to change the font size of, a "Text Edit" dialog window will open. Change the properties of the text you want to change such as line width, text height, character aspect etc. You cannot change the font size directly as such.
To make gerber files go to Option menu, click on Post Process Settings ...., a Post Process spread sheet will open, click on the post process properties you want to change as per your requirement and close the spread sheet.
Now click on Auto menu, click on Run Post Processor. Gerber files will generate.