Can't change Dock size
After deleting the "/Users/YOURUSERNAME/Library/Preferences/com.apple.dock.plist" file you will need to reset the Dock for it to take effect. You can do this simply in a Terminal window with killall Dock.

You could also just try editing the plist file in a text editor. The tilesize parameter is the one that determines the dock size. It should look something like the following.

&lt;key&gt;tilesize&lt;/key&gt;
&lt;real&gt;50&lt;/real&gt;


You should also be able to resize the Dock by clicking on the dividing line between the application tiles and the file/folder tiles.

If resizing just doesn't work at all, there is a way to prevent changes to the Dock by making the size "immutable". If that has happened you can re-enable changes by entering the following commands in a Terminal window.

defaults write com.apple.dock size-immutable -bool false
killall Dock