Nikon D810 - Manual (Page 108)

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Image size for JPEG and TIFF images can be set by pressing the
T button and rotating the sub-command dial until the
desired option is displayed in the control panel.
T
button Sub-command
dial
Control panel
A The Image Size Menu
Image size for JPEG and TIFF images can also be adjusted using the
JPEG/TIFF recording > Image size option in the shooting menu
(0 290).