Look up into the track. If there are no screws in the track, and the piece seems to be a molded piece to the actual fireplace, you will not be able to remove the track.
In this case, measure to the very bottom edge. For this picture, it would be just about where the red 9 is, or the spot this arrow is pointing to.
If you look up into the track and see 1-3 screws in it, use a screwdriver and remove those screws. With just a little work, that track should pop out. This will allow you to now measure to the top of where that track was - where the red arrow is pointing to in this picture.
Most pre-fab fireplaces have a manufacturer's tag on them. Getting the information from that tag can help us to make sure we're getting the right type of door for you.
Some fireplaces, like the one here, have to be measured differently than the others. With this type of zc fireplace, you have to measure to the absolute outside area of metal - top and bottom - left and right. This is not the "norm", but there are ones like this out there. That is why it is always best to send us a photo so we can help you.
Please DO NOT tear your fireplace facing entirely off like this. We cannot legally sell you a fireplace door for a fireplace that is in this state. This is dangerous and could compromise the entire structure of your fireplace. The only part that you should have to remove is the door and the track for the mesh curtain. Before removing anything, please call us and we can walk you through the process.