Have you talked to your HVAC contractor about how to integrate a fireplace into their plan? Heating the room that has the main house thermostat (or nearby) by using a fireplace can make the other rooms much colder.

Will you be putting a tv above it? If so make sure your fireplace insert is wider than the tv so the proportions look good. We messed this up.

We will not be putting the TV above, but it will be on the same wall.

Just think through sizes of things and what will look best for sure

It is not just about size of firebox but how it will affect other things in the space . Why are using it for heating ? I mean do you have regular heating and cooling and this is just additional? A good fireplace store will help with what you need but the visual is up to you. What is your style in the space , how often will you actually need a FP to heat? What brand are you looking at ? How will your funiture be layed out , I guess the real question is why a FP in the South