Don't know if the following dimensions will be of any help to you ?
We've got the VW supplied, I/R system with 6" screens fitted and I've just run a tape measure across them :-
Screen = 152mm x 86mm & outer frame/mount = 190mm x 127mm.
At the point where the headrests taper slightly towards the top, there is only 15mm of material left either side of the frame. However, ours are fitted fairly high in the rest where the material is thinnest.
Depending on how slimline the system is that you go for, you'll probably struggle to fit 7" without losing the rigidity of the sides of the headrests.
PS. Just checked with my wife and she reckons that extra inch isn't as important as you might think