That suggests that the aspect ratio of the image is different to that of the photo paper, so you'll either get some cropping at the bottom or some white space at the edges.
I never use the 'direct photo printing' option that you're using. I much prefer to open an image file in a decent image viewer/editor (such as Photoshop, GIMP or Irfanview) and print from there. (Alternatively I'll open a word-processing program or DTP program, import the photo into that and then print from there). I get much more control over the output as a result.
I'd normally take a look at the user manual to see if I could find anything to help you but annoyingly (rather than making a pdf file available for download) Canon has locked all the information away in an executable file that I'm not too keen to run on my own computer.
Simply opening the image in some suitable software, and then printing from there, should solve the problem for you though. If you've not already got Irfanview on your computer I strongly recommend it:
https://www.irfanview.net/
The print options there enable you to adjust your images to get the best results from the size of paper that you're using.