UPDATE:
New link for the icons image size by apple.
https://developer.apple.com/ios/human-interface-guidelines/graphics/image-size-and-resolution/
Yes it’s True here it is Apple provide Official documentation regarding icon’s or image size
you have to set images for iPhone6 and iPhone6+
For iPhone 6:
750 x 1334 (@2x) for portrait
1334 x 750 (@2x) for landscape
For iPhone 6 Plus:
1242 x 2208 (@3x) for portrait
2208 x 1242 (@3x) for landscape
For more info regarding Images and it’s resolution this is best ever helpful post
For setting images size for controls you can set 1x @2x and @3x like following: