Selling medium-sized prints, what is the safest aspect ratio and matting dimensions?

"artsy" photos like landscapes, or photo of a wave or smoke.. Vertical or horizontal. Why is 11x14" famous? If you could only choose 1 size/aspect to sell photos in, what would it be?

I want to offer just 1-2 sizes to limit decision making from the buyer's side, along with corresponding matting, and black frame.

Also, what is your markup in terms of %? For prints-only, would 2x the cost of printing be good or is that too low?