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I checked your from and noticed that the font size is the same when opening it on the Form Builder and loading the direct link, the current font size is 12px.
Loading the direct link:
I have also checked the custom style that you have added in your form and there is not any code that could be affecting this, or perhaps you noticed this and removed it; however, if you want to change the font size for your form you can do it by opening the preferences:
Then you will be able to change the font size for your form:
Other way to change the font size is injecting this CSS code:
This guide will help you to inject the code: http://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes.