What kind of fireplace is only 24" wide?

I would keep the hearth close to the ground.

You need to plan for the whole room including furniture placement, don’t just plunk the FP in the middle of the longest wall. Map out the room including circulation paths, furniture placement, TV etc. (Hopefully not over the FP if the firebox is only 24”, that would look very disproportionate & usually too high for comfortable viewing) The best place for the FP might be off center, or a different wall than what you had originally thought. Once that is determined, then your over all home design/style should help you with hearth etc Great pic provided by Beverly (unless the FP manufacturer has guidelines that need to be followed?)

What howiestyle said, 100%.

You should build the room to measurements of your furniture preferences AND the size TV ( or minimum) you will put in , unless this is a dedicated “ parlor” with no TV allowed.

I recommend you set aside a few hours to read HOUZZ posts on “ where do we put the TV”; “Help, my sectional is too big/ where to put sectional “ ; “ how to arrange my furniture in new house” ( usually this is an open plan with fireplace). Obviously I don’t know the exact search terms that will get the right topics and photos, but you get the idea.

Plus , focus in on how terrible are many of the furniture arrangements, distance from TV, no tables to put things on, walkways right through conversation area , disastrous- looking fireplace + TV walls. I’m not exaggerating, and this will be you if you just build some LR and then try to figure things out later. Then maybe counter with some searches to find the opposite, to find good examples of room layouts, options for different groups, focal points— often these might be pro or designer stories, not just HOUZZ but other sources, magazines or decorating books ( can view these through your library).