Canon EOS R3 Advanced User's Manual page 438

2.
Move the AF point.
Press the < > button, use < > to move the AF point into position
for focusing, then press < > (but note that with some lenses, it may
not move to the edge of the screen).
To center the AF point while using < >, press < > straight in.
You can also focus by tapping a position on the screen.
To center the AF point or Zone AF frame, tap [ ] or press < >
straight in.
You can resize Zone AF frames used for Flexible Zone AF by pressing
the < > button. Adjust the horizontal size with the < > dial
and vertical size with the < > dial, then press < >. To restore the
default Zone AF frame size, press the < > button.
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