People are commonly used to seeing prints in 4x6, 5x7, 8x10, and 10x20, however there are so many other print sizes and even custom prints to any size you want that a placed in custom created frames.
It is the image that is most important, not the crop size!
When your photographer edits your photos they will be cropped to a ratio that best displays that image and you should generally pick a print size that goes with that ratio versus cramming that image into a print size that does not go with the ratio.
A head toe portrait image if commonly cropped to a 2:3 ratio that fits a 4x6 print perfectly as it is. A larger print of that ratio would be a 8x12 print and would fit perfectly. You may be tempted to crop it to a 8x10 and cut off the top of the head or some of the feet or bottom because you don't know the 8x12 frame exists and you are used to seeing 8x10's but don't do it.
Preparing for a layout:
If you are having your photograper design a framed layout for your wall it will be predetermined what size image to where and which ones before they are completed and they will be in the correct ratio for that layout. If you are doing it yourself you will have to let your photographer know what images you need in which sizes so they will be in the ratio your require when they are edited.
Cropping off logos:
(Don't Crop off logos, order the right size and get the right size frame, or ask the photographer to resize the image)
Dowloaded images will have the photographer's logo on them and has to stay on the image and should not be cropped off.
Logos are typically small and in the right lower corner but can be placed elsewhere if needed for a specific image.
Prints ordered from the photographer's web site may not have a logo on them.
The Biggest Take Away here is: Generally speaking Order the size image that fits the ratio of the image.