As in placing in the editor or actually making your own?
For placing in the editor, grab one from the decal folder in the library, and place it on the map. Notice the handle to grab it easier.
Adjust it like any other object with the widget. Including scale.
Now if you want to make your own, you make them in a paint program and the normal ones are 16 celled on the decal sheet.
Open one of them in a paint program and you will see how they are laid out. They are run in order by the shader from top left, then to the next line repeating this till it gets to the last one, then repeats.
Duplicate a set and rename it and then modify it. If you want a different number of tiles in it, then make your texture sheet as so and change the value in the text file in that folder.
You may have to adjust the shader cell numbers to keep the decal from scrolling, instead of animating from one position, depending on the number of tiles you use.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit