Chances are, if you can plug it in, then you have the right size. HOWEVER, you need to verify that the wire colors stay in the same orientation from one end to the other of your extension cable. Some cables will tend to cross the colors - wouldn't think that should be the case on an extension, but worth taking a look to see.
I've made my own cables using Cat5 and the crimp-on ends. They work very well for extending between the AC3 and DC8/DC4.