This is why I am not a fan of interactive object creation. In the poly menu at the bottom is a check box if you uncheck it, interactive object creation will be turned off and a unit primitive will be created at the origin.
However, if you do chose to use it then it is easiest to create the shapes from one of the orthogonal views as dave suggested.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675