When you are selecting a fireplace mantel you need to focus on exactly what you want the mantel to do. The general guideline is that the mantel should be the transition between the brick or stone surround and the fireplace wall.  The mantel should frame the surround.

Here is an example of a mantel that just doesn’t work because  it is simply too small. When you have a large fireplace surround like this a mantel shelf is usually a great way to accent the fireplace.

Here are some mantel shelf options.

Problem Solved.