Form elements take their properties from the page/title properties for text font and size. you can change this either at title level or page level in the properties tab. If you want to do it just for one page you'll need to untick the 'inherit' box first.
NB: If you do it at title level it will ask you if you want to update all default text in your title based on new settings so watch out for that if that's not what you want.
