Nikon D780 DSLR Camera (Body Only) - Manual (Page 143)

97 Camera Controls
Choosing an Image Size
During viewfinder photography, image size can be adjusted by
holding the
X ( T ) button and rotating the sub-command
dial.
Choose from [ Large ], [ Medium ], and [ Small ]; the selected
option applies to JPEG photos. The physical dimensions of the
photos in pixels varies with image area.
Image area
Image size
[Large] [Medium] [Small]
[ FX (36Ч24) ] 6048Ч4024 4528Ч3016 3024Ч2016
[ DX (24Ч16) ] 3936Ч2624 2944Ч1968 1968Ч1312
[ 1:1 (24Ч24) ] 4016Ч4016 3008Ч3008 2000Ч2000
[ 16:9 (36Ч20) ] 6048Ч3400 4528Ч2544 3024Ч1696