I just finished one for a customer. It was to be painted, so I made it out of MDF. I had the 2 columns on the left and right, then the horizontal portion which ran across and bolted to the top of the 2 columns.
Each column was 34" high, 9" wide and 9" deep. Since they were so deep, I made both sides as well as the front of the columns raised panels, with the bottom raised panel twice as tall as the smaller one. With raised panels on all 3 sides, it looked very nice.
The top portion was huge (per the customer) -- 74" wide, 9" deep, and 17" tall. I thought the hieght should have been 9", but she wanted a high mantle, so there you have it. The front had 3 panels, the ones on the left and right were twice as wide as the middle panel. The sides of the horizontal box were also 9" deep, so they were raised panels also.
It was mammoth, and I'm a bit biased, but it looked great and the customer loved it.