Canon EOS-1D X Mark III Advanced User's Manual page 164

2.
Set the image size.
Standard/Auto switch card/Record to multiple
For RAW images, turn the <
< > dial.
Press <
Record separately
When [
to [Rec. separately], turn the <
press <
available for RAW images.
Select an image size on the screen displayed, then press <
> to set it.
Record func.] in [ : Record func+card/folder sel.] is set
>. Note that recording separately to
164
> dial, and for JPEG images, turn the
> dial to select [
and
] or [ ], then
is not
>.