Measure your stocking height. The bottom of your stockings should be about 1-2 inches above the fireplace cavity. Thus your mantel height will be as high as the stockings sit after adding 1-2" above the fireplace opening.

First check with the fireplace manufacturer. The installation instructions will have minimum required clearances for the mantel. They usually are dependent on how far out you want the mantel to project. Once you know the minimum clearance then you can adjust the height based on how tall you are, what you plan to put on it, and what material you are using. Sorry to be more vague than I'm sure you want, but please do start with the manufacturer's recommendations.

Thanks, we plan on only projecting 6 inches. We are 5'6" & 5'10".

There is a few factors should come to play like size of the room, size of the fireplace, size of the heart, etc.
Usually the min height is 54".

that was what i was thinking too. Thanks