The wall actually looks too small for a fireplace that will be in the correct scale for the room, can you post more photos of the rest of the room?

That looks mighty narrow, so you may have to see what's available with a box that small. It sounds like a gas fireplace is what you want, since most of the others you mentioned create no heat.

Yep, I realize it is rather tight -- not necessarily trying to heat the whole room, just wanting a little ambience

Do you have any blank walls in the room??? add a wall-hung unit?

Along with the additional pictures could you post room size and opening placement. It is a bit hard to tell in the picture how you would arrange furniture and if the fireplace would add or just look out of place. The wall does look too small in the picture, but it could be an optical illusion.

No, just this wall-- this is a sunroom with floor to ceiling windows on three sides

Wall is way to small for a fireplace to look good even if you could find one small enough. You also have to take into consideration venting.

unless you make the entrance to the left of the wall smaller? its pretty inexpensive to make openings smaller, bigger is the pricey one.

if those two openings go the same place you could close one completely. the room is huge and doesnt need to be more open.

Those walls look to be brick, so I'm guessing this sunroom was an addition to the home and they were exterior walls. Since they're painted white, finding the same brick style and painting it white will be no problem as far as matching goes. I would make both doorways smaller and thus, the wall you want for TV/FP larger. Consider installing some glass French style doors there as well if you want to give the dining room a more intimate and warm feeling, or you want to delineate between a casual sunroom and the more formal dining room.

Daniel Island Rhoden Island Living Room · More Info

An ethanol fireplace would be a beautiful and easy option for your fireplace. It uses an environment friendly, clean burning fuel that only emits small amounts of Carbon Dioxide and water vapors that is completely safe to you and your family. It requires no venting system of any kind. You can find fantastic options at Ethanol Fireplace Pros.