I don't understand what you meant with "getting it wrong".
I can change the resolution once I am inside the session, and the change is correctly saved. If I log out (or reboot) and log in again, the screen resolution "inside" the session is still correct, as I saved it before.
My question is about the first screen, before the session itself, where the user should select the desktop environment (in my case, I have Xfce only), type in the user and its password so to log in. While in that screen, the screen resolution is not the same as the one I selected in the session itself. Once the session begins, the screen resolution is indeed the correct one. I want to establish the same screen resolution for the first (log in) screen.
I am unable to find any info about this, whether it is about the log in screen resolution, or about the LMDE Xfce log in manager itself (I can only guess it is gdm, but I can't confirm this with any document).
I hope I was clearer now. Links? Documents? Instructions? Is this happening only to me?