Which political theory has been interpreted to mean that a president can shield critical information from congress as a result of the separation of powers?

What you are referring to is called executive privilege. This privilege gives president the right to withhold information from the legislative branch that he may consider to be classified. That is the simple explanation but it is a bit more complicated in reality.

What was the length to begin with?

Ibelieve the answer is  battle of saratoga but i could be wrong