How to change _greeter_ screen resolution?

[Michael Thibault]

Minor problem: the greeter screen resolution is such that I can't change it
using Ctrl-Alt-+/- so that the greeter is recognizable. I get only two
resolutions at that point, and neither is particularly useful - one
completely buggers the screen and the other renders everything in
triplicate vertically. Fortunately, the machine is set up to autologin
after a few seconds, and the session resolution is fine (and even
changeable). Which file(s) need(s) editing (and how) so that I can change
the greeter resolution to something that displays properly? I have looked
for the information, but it eludes me.