Yes, the chimney is masonry. Might 36" high be better?

Very squat I think. 36" is probably squat too given the width. I think it would give it a prefabricated look, which would be a shame since it is not.

Ours are 36" wide by 31.5 tall. They look proportional to me.

A 36" wide X 32" tall fireplace is very elegant and affordable in terms of firebox and chimney size. A 48" wide fireplace would be squat looking at a height of 32" and using the same proportion as the smaller fireplace would make the opening 42" tall which is a 70% increase in the opening area so the chimney would be more expensive.

But it all depends on the style of the house and the size of the room and how much of a central feature you want to create.

I prefer to use the smaller fireplace with a large granite surround and high mantle with a hearth that you can sit on or one that is flush with the floor.

A Rumsford fireplace is more vertical and may look better. It should perform better too! Good luck1

PS: Google rumsford fireplace for details and data.

How tall is the ceiling?

If the room is large w/ high ceiling, I suggest 48wide x 48tall; I'd go with 48x32 if the ceiling is no too high.

If the room is smaller than 19x20+/- and the ceiling is lower than 12', then, 36" wide X 32" is better.

The ceiling is 10 ft high and the room is 36 x 25. The house is a Georgian style.

Will there be a tv over the fireplace?

no tv, this will be more of a formal family room.

I second a rumford fireplace....they are so good looking and more efficient.

Here is a link that might be useful: rumford.com

Annie, what an adorable photo of your children. It's too late to do the Rumford for the family room, but maybe they can do it in the keeping room. Do you need to have a kit for this?

48x32 for that room is my suggestion.

there is a fireplace on the covered patio that is behind the fireplace for our keeping room so i'm not sure if we'll be able to do a Rumford there. My builder is checking on it for me.
for the family room fireplace the mason suggested 48 x 36 to prevent smoke coming into the room from such a large firebox.
thanks.

A Rumford is just a fireplace that roughly adheres to the proportions recommended by Count Rumford who was interested in the radiant heating qualities of a tall shallow firebox rather than the appearance. They are usually as tall as wide and sometimes they are so shallow that wood must be stacked vertically against the back wall instead of horizontally.

I have seen examples where the shallowness caused excessive smoking and wood embers to roll onto the hearth.

A 48" firebox opening with a height taller than 36" should not cause smoking; smoke is controlled with a properly formed smoke shelf and the correct dimension from the damper throat to the face of the firebox. It's an esoteric an art.

Here is a link that might be useful: rumford fireplace info