Miranda, if you compare proportions of your sensor size, meaning hieght to width, you will see the difference.  Divide the height by width and get a number, the ratio of  the dimensions.  Then do that with the standard sizes of photos.  Those with the same result will print with the same proportions.  The others will not.  Still, these may not fit your cropped image.

What to do?   Crop the image like you want it to be.  Open a new canvas (ps   file>new) and make the new canvas a bit larger than the cropped photo. (Use a white flood fill if you want but if saved as jpt it will fill anyway.)  Copy and paste your image onto the new background and crop the new background to the standard size.   You will have fill space (padding) around your image but this will print the right size.  If matted when framed, the mat may be cut to fit the image just right.

There is likely a much easier way but this is easy enough and works with the local and on-line printers who stick to standard sizes.