Nikon D780 DSLR Camera (Body Only) - Manual (Page 184)

138 Shooting Settings
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Choosing an Image Size
During viewfinder photography, you can choose the image size
by holding the
X ( T ) button and rotating the sub-command
dial.
A The Photo Shooting Menu [Image Size] Option
Image size for JPEG images can also be adjusted using the [ Image size ]
item in the photo shooting menu. Use the [ Image size ] item in the photo
shooting menu to adjust image size during live view.