Today I wanted to take a minute to explain how choosing a photo size may affect your photograph. When you choose a size, please remember specific sizes will affect the eshetic of your photograph. Depending on what size you want (more likely than not) a portion of the photograph may have to be cropped.
My camera (which happens to be a DX) naturally captures (crops) photographs in the ratio of 4x6 (8x12/16x24)...so naturally those are the sizes that work "best" with my photographs. My goal during a shoot is to allow some space around the subject to allow for cropping since the most popular sizes tend to be in the ratio of 4x5 (8x10/16x20/etc). Below are examples of how cropping will affect a given photograph.
The photograph on the left is the "original". This is the 4x6/8x12/16x24 (and so on) ratio. You will notice that there are no body parts cropped out whatsoever. For the photograph on the right, I have cropped the above photograph to the equivalent of an 8x10. Notice how their feet are starting to get cropped out? That is how your photograph can be changed. Granted, the main focal point is still included...there is still noticeable change.
Here is another example. Again the first is a 8x12 crop and second an 8x10 crop. Its amazing how much more "boxy" the 8x10 looks, isn't it?
Also keep in mind that FX cameras are a completely different ball game. This is just a quick guide for my clients as well as photographers that are currently using a crop.