Is there a formula for graphics dimensions?

I understand these are the dimensions for iPhone graphics:

Device Points Pixels Scale Physical Pixels Physical PPI Size
iPhone 6 Plus 736x414 2208x1242 3x 1920x1080 401 5.5"
iPhone 6 667x375 1334x750 2x 1334x750 326 4.7"
iPhone 5 568x320 1136x640 2x 1136x640 326 4.0"
iPhone 4 480x320 960x640 2x 960x640 326 3.5"

However for example, is there a formula to have graphics relatively the same size, in reference to the device.

For example, if on the 4 I have a graphics that is 50x50pixel, what would be the formula for to see what size I would need it on the 5,6 & 6 plus?