What size should can TV be over brick fireplace?

No larger that the fireplace opening itself... or find another wall (or room) where you can put BIG one

I always buy the largest I can afford that will fit in the space allotted. Never had one that I regretted because it was too big.

If you want better advice, send along a photograph of the fireplace.

I think as long as the tv isn't bigger then the fireplace and they balance each other then it'll look nice!

Ok, this is where builders code comes in! Nothing combustible can be any close to the firebox then 36” . Which the heat alone any closer could cook your TV set! If you use your fire place? 12” on the sides of the fire box to combustible as well! I’m a contractor and also installed Woodstoves for a living in the past. So I know the codes well!

I'm of the school that the TV should not be over a fireplace. They compete with each other and neither wins.

You can put up any size you want as long as you can secure it safely in the allotted space.