If you go to any store that sells ready made frames you'll find they are sized to hold 4x6, 5x7, 8x10 prints. The crop tool in our editing packages often defaults to these standard sizes. That's fine if you plan on printing your photo and using a standard frame. Most of us don't print many of our photos any more. They end up on a website somewhere. Yet, we tend to use the standard crop sizes. If the photos are going on a website they can be cropped to any width and height that works best for the photo.
Here's an example. The first photo is not cropped at all. This is the way I took it.
None of these really work for me. There is too much wasted space at the bottom and top of the photo. This does not add anything to the photo and a good rule of thumb is to eliminate anything that doesn't add to the composition.
Here's the crop I like best.
If I was to print this I would have to go back and find a crop that either fits a standard size print/frame or go for a custom photo mat and/or frame. In this case it's pretty close to a 10x20, which is a size that I can get printed. If you're looking for a frame for a 10x20 print you may have to go somewhere besides Walmart and Hobby Lobby. Better yet, forget the frame and print as a canvas wrap. I've done three of those now and love them.
This photo is available for purchase online. It's ready to print at 4x8, 5x10, 8x16, 10x20, or 12x24. Many of these sizes are available as metal or canvas prints.