Currently, changing the font size or format of the form title isn't directly supported. There is a workaround, though!
In the Header & Profile section of your form's settings, you can set the Form Header Title setting to "No Title", and then repurpose the Form Header Description to set a title. In the description, you have control over the formatting, so you can adjust the font size as needed by using the different heading options (H1-H3).