Is it possible to develop an app only on specific device size?
Hope this helps you.
As of May 1, all new iPhone apps and app updates submitted must support the 4-inch display on iPhone 5. All apps must include a launch image of the appropriate size. Learn more about iPhone 5 support by reviewing the iOS Human Interface Guidelines.
And as per App Store Review Guidelines

2.10
iPhone Apps must also run on iPad without modification, at iPhone resolution, and at 2X iPhone 3GS resolution..Please refer this

Else Apple may reject your app if app is not as per apple review guidelines