Jan, the sizes on FAA are based on even numbered dimensions on the long side. The width of the short side is based on the aspect ratio (length:width) of your uploaded image. The beauty of this system is that any print can be matted to fit any sized frame, within aesthetic limits. In short, 'standard' sizes are not spoken here. You can upload 'standard' proportions, but keep in mind that 5x7, 11x14 and 16x20 (for example) are all different proportions, and no one upload will satisfy all three. Just crop for the image and let FAA do the framing and matting for the customer. They have a long history of making it all work.
Just look at the recently sold images, bare, matted, and framed.