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I usually change resolution whole X-wide by altering /etc/X11/xorg.conf because it's just so easy and should affect any desktop running X (though I'm not sure if the desktops like Gnome and KDE nowadays change the resolution on-the-fly themselves). In the file scroll down and look for lines that describe resolutions for each colour depth in the form of "1024x768" "800x600" "640x480" etc. and alter those lines (usually every one of them, to get changes for all colour depths). The leftmost resolution in the line is used if possible, others are available in case needed. You may also try switching the resolution with CTRL+ALT++ and CTRL+ALT+-.
EDIT: the keys I mentioned work while in X. After editing xorg.conf the changes take place only after you restart your X: this is easiest done by pressing CTRL+ALT+BACKSPACE and logging in again.
Distribution: Debian Wheezy/Jessie/Sid, Linux Mint DE
AFAIK the resolution is determined by X, not by KDE or Gnome. At the most you can set it from within KDE or Gnome, but it is really X you are adjusting.
If you are referring to the login dialog, in KDE there is a setting which let you choose the size of this dialog. In KDE you find that in KDE Menu > Control Center > System Administration > Login Manager.