Nikon D780 Reference Manual page 184

❚❚ 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.
138 Shooting Settings