When purchasing portraits, you have many size options to fit your portrait needs. But, not all portrait sizes have the same dimensions. Even though the proof may look a certain way, when you get the actual print, it may be cropped slightly different. In some photos, you may not notice any difference, but some may look a little more "zoomed-in" than when you originally saw the proof.
When I shoot portraits, I try my best to leave enough space around my subject so that cropping won't affect the overall look of the portrait. Every once in a while, though, certain factors may prevent me from being able to do so.
Below are some portraits cropped at different aspect ratios to show you how the different portrait dimensions look.
2:3 Aspect Ratio
Portrait sizes: 4X6, 8X12, 16X24
4:5 Aspect Ratio
Portrait sizes: 4X5, 8X10, 16X20, 24x30
Both 5X7 and 11X14 are also common portrait print sizes.