The problem here is that you are thinking in "Standard" sizes which no one should be doing anymore.
Forget standard sizes of 8x10, 11x14, 16x20, etc.
The image tells you what size it needs to be. Either you shoot for the composition of the image you were trying to get, or shoot so that all of your images can be cropped to fit those outdated sizes.
The main reason people try to make their images fit to those sizes is that they are cheap and just want to go to wally world or somewhere and buy the cheapest frame they can buy.
Stop degrading your images like that just so you or your customer can save a few bucks on the frame or the mat.
If your work can't handle the extra cost of doing it right, then you might want to rethink what you are doing with a camera.
You'd be amazed how much people are willing to pay if the piece looks right and not crammed into a standard frame.
...and yes, I'm amazed the mass producing framing industry still insists on making frames to fit photo sizes that no one shoots.
I'm always having to show my customers how the image they have just won't fit into a "standard" frame and what would have to be cut off to make it fit.
My favorite response is.. well, can't you just run a strip of mat board along the short sides?
To add my answer to the op's question about size.. Better quality poster catalogs will tell you the image size AND the paper size. Always tell the customer the actual size of the image. They couldn't care less what size paper it is printed on.