Have you considered electric? They are great for this type of application.

Orion Multi Electric Fireplace.

Available at Hansen Wholesale

We also chose linear electric for the following reasons.

- No heat produced. Can enjoy the light show all year round. Kids can approach it without getting burned. You can run it for heat but it is optional.

- No yearly maintenance. No chance of gas leak. No fumes to exhaust.

We got a Dimplex because our neighbor got one and I was able to see the flames in person.

https://www.dimplex.glendimplexamericas.com/en-us

It is my favorite thing about our fireplace remodel.

Originally we put in a 77" OLED TV but ended up downsizing to a 65" OLED TV. Turns out for us there is such a thing as a TV being too big.

For the length of the fireplace it is personal choice. Some like to say it has to be wider than the TV. Personally I don't like that look so we chose a 48" and are very happy with how it looks.

Please be aware that you have to watch out for clearances on your fireplace to TV spacing. You don’t want to melt your TV. To me, this would be another reason perhaps not to go quite as large with the TV.

You can use a heat kit to move heat away, so the tv can be only a foot or so above the fp without a mantle - like this https://www.fireplacex.com/products/accessories/coolsmart-tv-wall/. Especially with a smaller room, make sure that the tv is not too high. Do all the math and read the installation manuals before buying anything.