Kiki, So happy to hear you are commissioning an artist for your daughter's portrait! Actually, an artist in general!!

If your f.p. is gas, there should be no problem with hanging the art above. But, you will need to have it professionally framed. This is very important if you want the painting to last. You will need to use archival mats and museum glass. The archival mat is so that no acids touch the w.c. paper. Even the tape used to hold the art to the mat needs to be acid free. The museum glass does 2 things: it will cut down on glare when viewing the art, and it will reduce UV rays hitting it, too. Sunlight is a huge enemy of any kind of art, too. And then, depending on the frame you choose, this is not cheap. Nor is it the place to cheap out. You may also want to look into lighting for the art. Although a cathedral ceiling and stone may be an issue...just something to think about.
Finally, you can't decide placement until, like you said, you know the finished size or orientation. Will the artist be providing sketches? Or maybe a *sneak peak*? I don't do portraits, but when I am commissioned, my process is always to present sketches, sample of colors, etc. Of course, once you unveil that beautiful piece of art, those things don't matter.
And, if you do hang the portrait over the f.p., and maybe add a mantle shelf, (that metal grate above the firebox is where the heat will come out of) the amount of stone may not bother you. It's always good to get your stuff into the space before making any big decisions.
Looking forward to seeing the portrait!!

paintergirl is right about the gas fp. Soot doesn't deposit on the wall above it, does it?

We have a huge original water color of our favorite place that hangs over our sofa, and we commissioned a pastel of my father and our two youngest daughters 22 years ago that hangs over our piano. Both were professionally framed as paintergirl suggested, under museum glass, completely sealed. Their colors are as vibrant as they ever were in our very suuny, south-facing living room.

Like pg says, this is not the place to go cheap; a professional framer will offer many good suggestions for framing your portrait. We enjoy our our paintings every day. It just feels good to have them in our home. A big thanks to artists like paintergirl, too, for helping make the world beautiful.

Sherry

Hey Shermann -- a big thanks back at ya -- for employing talented people!! I admire those who do portraits. It's a talent all its own.
Maybe we can do a post on commissioned artwork for our home sometime.

thank you very much for the advice paintergirl,
i'm glad that the gas fp will be okay.

i plan to use the same framer that the watercolorist uses for her work, i know it will be a little spendy but i'm saving up for that too.

Hi Kiki, I just wanted to let you know that I did damage a portrait over my gas fireplace in my last house. It wasn't under glass, however, it was an oil painting that sustained some warping from the heat not soot. In my present home I do have a well framed watercolor over my gaslogs but I take down the picture when I put the logs on, not very convenient if you use the fireplace a lot but I didn't want to take the chance of ruining another piece. We do have a wood mantle but it gets hot and the wall above does too. Yours may be different; just feel the space with your hand to see if it heats up.