As Zearc said - some screens report using EDID (if you have turned this on) the wrong screen size - eg 16x9 which is then interpreted as 160mmx90mm!
If you have turned on EDID, then you could try using UseEDIDDPI - false to let it use the default (75dpi), or explicitly set it using UseDPI - 100
Alternatively, if you haven't turned EDID on, then you could try this as the screen might actually report the correct DPI and this would fix it as well. Change the line "UseEDID" "false" to "true".
EDID will only work with connector types like VGA, DVI and HDMI, not composite or other analogue ones. All these settings are in your /etc/X11/xorg.conf - you can edit this by connecting to your core remotely from another computer using ssh (download PuTTY) this will get around your issue of not being able to see the screen. Its a text terminal program. Easiest to do this from the Internal network. You might want to install Joe as your text editor:
sudo apt-get install joe
Oh, and if you installed with the DVD, the username and password should both be linuxmce