try this link, maybe it will answer your question

Here is a link that might be useful: fireplace mantel min clearance

I used a table similar to that one, except mine came with my fireplace insert. That gave me the minimum height based on the width of the mantle, and then I decided how much higher I wanted/needed it. I've got 10 foot ceilings, and I needed to decide if I wanted the mantle to be lower or higher than the midway point. I ended up going with higher, so that the mantle ended up being about 5.5 feet high total.

I'd only use the numbers on cynandjon's link if your fp builder doesn't provide you with figures for your specific model-- I know that with gas burners, the clearance varies by BTU.

In artistic terms, I think a higher, wider fp is more appropriate for larger spaces. In fact [and despite the fact that the trend seems to be running the other direction] I like to see fps in tall spaces lifted 12 to 16 inches above the floor so that the flames are at eye level when you are seated. A nice bonus to this is that the hearth can make nice extra seating when needed. Makes the fp seem much larger, more of a centerpiece.

But ultimately, its all about scale. The mantle height that would work well with a large firebox might seem a little strange with a smaller one.

I'd get some graph paper and do some scale drawings.

And you might want to be looking at some art to go way up high on that wall as well.