The help message when taking an Area screenshot says that arrow keys move in 1-pixel steps. On my 4k screen, the move the selection in what looks like 10-pixel steps.
Working on notebook with openSUSE LEAP 15.4. Resizing a rectangular can't be done with arrow keys. Only moving this box in greater steps is possible. But not with constant steps for- and backward. This functionality was given in previous version and seems to be solved again, when using openSUSE Tumbleweed.
The help message says the arrow keys move in 1 pixel steps if you hold Shift.