You can change the webcam’s resolution via the cmd line on OctoPrint by
sudo vi /boot/octopi.txt
And adding this to the file:
camera="usb"
camera_usb_options="-r 1280x720 -f 10"
You can get a list of the supported resolution via (may need v4l-utils installed):
v4l2-ctl -d /dev/video0 --list-formats-ext
and then reboot the pi