The New Layout: How to change the font size?

Unfortunately that options it's not yet available on "Single question in one page" layout. I have submitted this feature request to our developers and you will be notified on this thread when this is implemented. We are sorry for any inconvenience caused.

Please note that we decide to investigate feature requests regarding font customisation. That’s why, we would like to know more about which style exactly you want to change, font family, font size, font weight, font color or all of them? 

We respect our users concerns and try to serve them in a better way. If you inform us as much information as you can, we will try to find a way to accomplish your needs.

In the meantime, We are happy to inform you that font family option is added to Form Designer for JotForm Cards.