To view some applications you may require more space on your Mac screen. If you need some space on your Mac OS X desktop you can hide the dock or even make it smaller or bigger on the fly if you are no comfortable by hiding it. So, it is very simple to quickly change the Dock size on Mac OS X and the method is as follows.
See on right side of your Dock there is a bright doted line which separates Recycle Bin & opened running applications from other icons
Just take your mouse pointer on those bright dots line and cursor will turn into a resizing cursor. Hold the mouse button and drag it upward to increase the size of Dock and drag downward to decrease the size of Dock.
So, this is the quick way to increase or decrease the size of Dock icons. See also Customize the Dock.