I see that you do not have a masonry chimney, but are going to use insulated stovepipe..
I am a bit concerned that this very tall fireplace will not generate enough draft to avoid being smoky. Was this designed by someone who understands fireplace technical requirements?
If so, I think it would be very attractive, and impressive in the room. The height would be very striking, and in scale with the large windows and high ceiling.
I went with a lower mantel, as I like to use it in a more functional way - to place a drink on, to rest a pair of insulated fireplace gloves on, to hang stockings from, etc.

Thank you sstarr! Yes, this was designed by our architect with our contractor who has built similar many times. The opening for the fireplace will be lowered, just the mantle is designed to be 6'3". I agree that it would be nice to display objects, hang stockings, etc. Maybe I should lower it by a foot to 5'3", or to somewhere in the middle to keep in scale with the room.

I think lowering the mantle is a good idea. For finishing, a uniform surface from floor to ceiling would be a good back drop for whatever you display on the mantle - marble, concrete, slate, barn wood (wait - can wood be used on a fireplace or does it need to be less flammable?). Looks like it'll be a gorgeous room!

Do you have a rendering or images of similar fireplace? What will the final finishes be?

Yes, if it makes sense for the fireplace opening, I think the somewhat lower mantel might work better for you.
Although we have a high ceiling, we were limited by the framing and panelling in the room, somewhat (below).

sstarr, you have a gorgeous fireplace! Thank you hdyer. I am considering concrete or slate like these photos, ppf. What do you think? I'd love any suggestions on how to finish the fireplace.

I love the concrete idea; would have used it myself, except we're in Vermont and this is local stone, from a quarry on our street.
I don't think you could go wrong with slate, either, as long as you stay away from the multicolored variety..

None of your inspiration pics have mantles except for the last. You might revisit you ideas about having a mantle at all.

Sophie Wheeler - thanks for pointing that out. I will let you know what I end up doing!

I was curious, so I modeled it. Guessed at dimensions. The second and third I used 55" and 90" for the upper and lower parts.

And I do like the idea of concrete.

Why not have the mantle at the same height as the cross-pieces of the windows? It will make a clean, unified line.

Simple but with a Statement!!!

ppf., you are so AWESOME! Thank you for the renderings! So helpful!

maryenidlareg, great idea!

Imondi Interiors, I love it. Is it easy to find in the Boston area?

Currently, the mantle seems too high for the scale of the windows. If you hang curtains rods up near ceiling the windows and fireplace will balance. But I do think that lowering the mantle or bringing it up near the top of the window frames would work much better.

If you go to AmericanClay.com you can check to see if there is a distributor there. You can always call American Clay directly they are great. The product is Awesome!!!

Maybe consider a gentle arc for the face? Image is looking down from above.

Another great idea, ppf.! Thank you bunnyllg...I think I will lower the mantle. I plan to do no drapes, just remote operated (battery) solar shades.

http://montanarockworks.com/stone_product_detail.php?name=Moose+Mountain+Ledge+Signature+Thin+Veneer&uuid=7A36F-4AB9

^The most stunning real stone I have ever seen. Maybe a mantel that is of similar construction to the lower chord of your timber truss? 6' top to the mantel is a pretty nice compromise...