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I took a look at your form.
There are 2 ways to fix this and I will give you the preferred one first.
Add Mobile responsive widget to your form Mobile Responsive - Form Widgets .
You can see a clone of your jotform with this widget added to it here: http://form.jotformpro.com/form/42615395795972
It will handle several different things for you automatically in order for your jotform to show properly on your visitors mobile devices.
The other way is to inject CSS code that will change just this issue:
@media all and (max-width: 300px)
You can see here how to inject CSS to your form.
If you link you can clone my form or add the mentioned widget by yourself and check how it works :)
Again, I suggest adding the widget only, not to use CSS, unless you do not want to add the widget for some reason :)
Do let me know how it goes for you.