Используйте фичи медиа для девайсов — device-width, device-height, orientation, device-aspect-ratio
iPhone 6+
media only screen
and (device-width : 414px)
and (device-height : 736px)
and (orientation : landscape)
and (device-aspect-ratio: 736/414) { }
media only screen
and (device-width : 414px)
and (device-height : 736px)
and (orientation : portrait)
and (device-aspect-ratio: 736/414) { }
ipad
media only screen
and (device-width : 768px)
and (device-height : 1024px)
and (orientation : landscape) { }
media only screen
and (device-width : 768px)
and (device-height : 1024px)
and (orientation : portrait) { }