Nikon Z9 Mirrorless Camera - Manual (Page 108)

108 Image Recording Options (Image Area, Quality, and Size)
Choosing an Image Size
Choose a size for new JPEG pictures. The size for JPEG pictures can be
selected from [
Large ], [ Medium ], and [ Small ].
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Choosing an Image Size
Hold the T button and rotate the sub-command dial.
A The Photo Shooting Menu [Image Size] Item
Image size can also be adjusted using the [ Image size ] item in the photo
shooting menu.