there are a few things it could be, but most likely your DPI settings are not being detected properly.... (when you are logged in, you probably have forced KDE to use a certain dpi for fonts (eg 96))
I have done the following on my Dell Latitude C840 that does not detect these things nicely... (cut and paste from my internal wiki... so formatting might be funny)
To get really nice looking fonts,etc, just update the dpi - edit the xorg.conf.
Add to the Monitor section
DisplaySize 355 230
Which is the x y dimensions of the screen in millimeters and Add to the Device section
Option "UseEdidDpi" "false"
Which will tell the server to ignore the EDID DPI setting, and use those specified.
You will need to change the displaysize values - they are my laptop measurements using a ruler... so measure your own (unless these work - 15" dell screen)
There are a few other tricks... but hopefully this will fix it