What size do I set the stage to for the same app to be viewed on iphone and ipad

It seems like such a simple thing, but I have searched all day online for the answer to this to no avail!

I'm about to attempt my first app build in Edge Animate, to be output through PhoneGap Build for different devices.

But what size do I set the Edge Animate stage at so that the same app runs on iPhones and iPads?

Do I just design the graphics to suit iPads (and what size is that exactly?) and do I also set Edge Animate to 'Responsive Scaling : both', so that it works on iPhones as well?

Thanks in advance.