Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 487)

Resizing JPEG Images
You can resize a JPEG image to make the pixel count lower and save it as a new image.
Resizing an image is possible only with JPEG /
/ images. Resizing is not available
for JPEG , HEIF, or RAW images, or for frame-grab images from 4K movies.
1.
Select [ : Resize].
2.
Select an image.
Turn the < > dial to select the image to resize.
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