The default options for the dock's icon size in System Settings are "normal" and "large". Is it possible to set the icon size to "small"?
You can hold Ctrl and scroll over the dock to change the icon size.
There is also a hidden preferences window for Plank. Hold Ctrl and right click on the dock to open a context menu, then select "Preferences".
You can also open Terminal and type in
$ plank --preferences
At the bottom of the window there is a slider to adjust the size of the icons.
Bear in mind, that elementary does not provide icons at arbitrary sizes, so your dock icons may appear blurry at some sizes.