Understanding Aspect Ratio
Photo files straight out of the camera are in a 3:2 aspect ratio-- meaning the size of the file's height by width. When I provide my clients with proofs, those files as presented are almost always in their original aspect ratio.
The most common prints my clients want are 5x7 and 8x10, both different aspect ratios thus requiring cropping that may make changes to the way your portrait looks.
If you wish to keep a 3:2 aspect ratio and have a print that looks exactly like the original file, then I suggest purchasing either a 8x12 or 16x24 print. 8x12 frames are becoming more readily available in stores and online.