iPhone device ownership statistics
The most recent data I've been able to find is for iPhones from 2015 and it's from Localytics.  The company states they...


  ...examined over 100 million iPhone devices. The data for this study
  represents iPhones that had active app sessions logged during August
  2015.


Here's the hightlights of the study, the details can be found here.


Here's some info from Unity3D based on their usage for March, 2016.  It inlcudes all Apple devices.  

Top on 2016-03:

iPhone 5S: 21.0%
iPhone 6: 13.5%
iPad Mini: 9.5%
iPhone 6S: 6.7%
iPhone 5: 5.6%
iPhone 6 Plus: 5.5%
iPad Air: 5.2%
iPad Mini 2: 4.6%
iPhone 4S: 4.4%
iPad 2: 3.6%
iPad Air 2: 3.2%
iPad 4: 3.1%
iPhone 6S Plus: 2.9%
iPhone 4: 2.7%
iPhone 5C: 2.6%
iPad 3: 1.8%
iPod Touch 5th: 1.3%
iPod Touch 6th: 0.9%
iPad Mini 3: 0.8%
iPad Mini 4: 0.7%
iPad Pro: 0.2%
Apple TV 4: 0.0%
iPod Touch 3rd: 0.0%


As a developer, I just use AutoLayout and not worry about what device my users have.  That seems to work for me but each app is different.