What headrests do I use in the back seat?

I was originally intending to install headrests with 7" screens in them, same as the front have but after much consideration have decided that they will only be installed for particular events with standard style headrest fitted normally. Now this is where the problem lies, do I use the smaller rear seat or larger front seat headrests?

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and in the car

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MartyTheTrimmer reckons I should use the larger ones and as usual I think he is right, What do you think?