Alcove off fireplace - size of air vent?

[Justice League]

So the fireplace will be turned into an alcove where we can add a nice vase and light as a centrepiece as we have had the olf gas fire removed.

We shouldn't block the fireplace up as it can lead to condensation so what size hit n miss air vent should I add to the top of the alcove. It will be hiddent from view.

I can go with a 9 x 3 or 9 x 6 vent.

 

[king of pipes]

9x6 would be my choice