I've got 6.5" ones in mine, and from what i can gather from the people that installed them, they're a pretty tight fit.
As terry said, be careful of the size of the casing too, cos these can be quite big, and will cause problems. Even though i have 6.5" screens, they actually are mounted in casings that used to be supplied for 5.8" screens
It's not just the size of your headrest though, it's all based on the available room inside the headrest due to the metal headrest supports.
If you need any more info, drop me a line