It is all a matter of opinion and also the proportions of the room and how the opening will look in relation to the rest of the room.........Where are the windows ? How high are they ? What about the furniture and the TV and how they relate to the fireplace.......

You might do a mock up with pieces of cardboard as your opening.........

Or if you have an architect or designer, you might have them draw up the interior elevations of the space showing your options.

The first thing is to decide is what is the primary viewing eye level. In most cases, the fireplace is being viewed from a seated position, so eye level is 40"-42" off the floor. I'd recommend placing the top edge of the fireplace opening at that eye level height, or a little lower, so that you look down into the fireplace a bit and don't see the inside of the top of the fireplace.

If the fireplace is primarily viewed from standing height, eye level is about 60"-66" off the floor, and you are faced with a choice. If the fireplace will share the wall with artwork or a TV over the top, you'll want to use the same rule as for the seated viewing area, possibly coming up a few inches if your wall is over ten feet high.

If the fireplace is a singular focal point and seen primarily from a standing position, I'd suggest placing the top edge at eye level, with some variation allowed to match horizontal sight lines in the area.

As per:

https://www.quora.com/What-is-the-recommended-height-from-the-floor-to-mount-a-gas-fireplace