Sometimes it is difficult to visualize aspect ratios between media of varying sizes. The following are charts show how varius sizes of media relate.
Photo Paper Sizes
4x6 maps direclty to 35mm film, 5x7 is nearly identical to 645, and the rest are enlargments of 4x5.
Common Digital Ratios
Most digital cameras can also use 16:9, but I doubt anyone uses that aspect for stills. In practice, 3:2 is the most common, and the widest aspect. Medium format cameras have a native sensor size between 4:3 and 7:6, which is an interesting ratio to consier cropping to even if it's not native to your camera.