I don't have a rule of thumb to share with you dimension wise, but I did want to share a photo of a mantel that's similar to your inspiration but is more substantial. I think this larger piece would better fit your large room and volume ceilings.

Your best bet would probably be to buy some trim in varying sizes and hold it up to decide.

I found a mantel I liked in a magazine and then worked with a trim carpenter to work out my version of it.

What style is your home? Can you take some current photos to show the new windows and a straight on view of the fireplace showing a little on either side please?

It seems to me that you have some important constraints there already in terms of the raised hearth and the width of the brick. How high do you want your artwork to go? How much difference do you want between the height of the mantel and the height of the wainscoting?

You might just get some painter's tape and newspaper and start blocking out some options so you can get a visual in terms of height and how it might look....that may help you decide.

But I'm presuming you are not DIY'ing the project...a good trim carpenter will be able to help you decide. When I worked with mine, we laid out the design on his work table with scraps of molding so we could get a nice visual of how high the mantel would need to be and how far out it would stick once you stacked up all the moldings...and in my case, how much the bookcases would need to be covered on the edges for the molding to die into.

Here's the before pic again
{{!gwi}}

Here is an "unstyled" picture with the updates. I plan to keep the compass rose over the fireplace and hang it a little higher after we add the mantle My DH loves the crazy coffee table made by a hippy by the name of Hondo so even though it is too busy, it stays for the time being.

{{!gwi}}

This post was edited by deee on Mon, Oct 21, 13 at 9:10

DH is my carpenter and as you can see, we have already played with tissue paper and wood scraps The pillows and topiaries are just parked until I find another spot for them. The curtains are one sided because the hearth extends about five inches under the window sill. All the curtains are hung low because the height is matching older windows on the other side of the room that aren't seen in the photos

{{!gwi}}

Here is what you see as you walk in the front door.

{{!gwi}}

This post was edited by deee on Mon, Oct 21, 13 at 10:38

Ah I see...

Well, you can look at your inspiration pic...common brick is usually 8" long, so it looks like they left about 10" of brick showing before they started the mantel, the first board looks about 8" high, then the trim piece...have you gotten the brackets yet? They look to be about 4" high, then a few boards on top of that and that sets your height.

Hard to tell in the pic, but it looks like they left about 8" of brick showing on the sides which looks to be about where the wider portion of your tissue paper is. I'd mark it out better all the way around and take a look...I think it will look fine.

You can look at enkeboll or van dykes for corbels...but they're not cheap.

Here is a link that might be useful: These guys may be cheaper

This post was edited by AnnieDeighnaugh on Mon, Oct 21, 13 at 10:05

Annie, thank you for the link and the comments I was hoping that someone would answer and pronounce that according to Trim Carpenter's Bible or some such book, "the height and width of the mantle shall shall not exceed blah, blah, blah".

Oh well, Maybe I will post the question at another time and not make it sound so much like a math problem.

Maybe I will mock up a mantle, announce that I will not change it and then see what kind of suggestions i get :)

Sounds like a plan....
People always have an easier time tearing down what you've built than suggesting things from scratch...

What a beautiful home Deee!
I've not time to read new comments as I'm staving and need to start dinner, but I wanted to add the virtual I did thanks to your head on photo.

justgotabe! DH and I are very impressed with your skills.
Does it seem to you that the surround makes the fireplace look a little squat? When you have a chance, will it be possible to take away the surround and beef up the mantle? Thank you so much.

I don't think it does, Whenever you have such high ceilings I expect the fireplace to be at least a third of the highest point of the ceiling your surpasses that.
I do like it better without the surround though.