the size in pixels so as to keep the same ratio, then
the portrait view and landscape view should be the same.
Take my screen as example :
Physical size : Physical Width (PhW) = 47.2cm, Physical Height (PhH) = 29.5cm
Pixels size : Pixels Width (PxW) = 1680 , Pixels Height (PxH) = 1050
Physical ratio : 47.2/29.5 = 1.6
Pixels ratio : 1680/1050 = 1.6
HTH