Honestly, it should be easier – then again, it’s Gnome, nothing really is easy.
Collecting a combination of various pages (this one finally yielded the right directions) it turns out one has to install imwheel and then adjust the ~/.imwheelrc file:
# .imwheelrc
None, Up, Button4, 3
None, Down, Button5, 3
Control_L, Up, Control_L|Button4
Control_L, Down, Control_L|Button5
Shift_L, Up, Shift_L|Button4
Shift_L, Down, Shift_L|Button5
The magic incantation is in the first couple of lines and the value (3 in the example) is what will change the scroll size: in my experience, the baseline is 3 lines, and the actual value is multiplied by the factor given here (in our example, every tick on the mouse wheel will cause a 9 lines’ scroll).
Finally, for the changes to take effect:
$ imwheel -kill
no need to sudo or logout.
Really cannot for the life of me understand why Canonical cannot add that control alongside the mouse speed setting in the System Settings.
