Can a larger republic, based on the principle of consent of the governed, sufficiently protect the rights and liberties of the individual states and people, or is a confederation the only method of securing such liberty?

With a larger republic based on the principle of consent of the governed, the general legislature has the most likely authority to make laws which will affect the liberties of everyone- - which no state can prevent. The confederation, it is likely that the individual states secure the liberties of the people. The only method of securing the liberties of the people is confederation, because it represents its citizens more closely than a large republic does for the entire nation.