We are often dealing with various print sizes though there are a few that pop up more often than others. In particular A-sizes. A-sizes tend to be popular due to the standardised mounts and frames that can be bought relatively cheaply compared to custom framing.
A few things to keep in mind when ordering an A-size print:
Will the image butt up to the edge of the mount window? If so, you'll need to add at least a 1cm boarder all the way around your image so the mount has something to hold on to.
Is your image file the right dimensions for an A-size? If not it may be a case of cropping your image to fit or going for a non A-size print.
Here's a helpful guide for standard A-sizes: