In Tolkien's Middle-earth, how far does Wilderland extend?

I was editing my answer to this question, in which I tried to describe the size of Wilderland, and found myself not quite sure what it included. Is Mirkwood part of Wilderland? Does it extend all the way around Rohan to the gates of Mordor? Does it go east past the Lonely Mountain?

I was assuming that when Gandalf and Aragorn explored it in search of Gollum, they didn't go into or past Mirkwood, nor east of the Brown Lands—an assumption I still think is reasonable. But the question still stands: Granted that "Wilderland" is necessarily (as I state in my answer) a vague indicator, is there a specific canon reference to how far it extends in different directions?