How big was the wilderness in which John the Baptist lived and preached? At Matthew 3:1-2 we read :

In those days John the Baptist appeared in the wilderness of Judea, proclaiming,  “Repent, for the kingdom of heaven has come near.”

One wonders how John the Baptist managed to get a lot of audience in the wilderness. It could be that the place was very much in proximity to human habitation, or was on the way to some centre of pilgrimage. 

I wish to know where exactly in Judea  the wilderness in which John preached was, and how big it was .
While the exact location of John's ministry is not known, we know that it was around the Jordan, with John 1:28 specifically placing it at "Bethany beyond the Jordan." The traditional location is places it just north of the northeastern corner of the Dead Sea. Jerusalem, the center of Second Temple Judaism, sits at the top of a verdant hill country. But as one makes the 10-mile downhill trip towards the Jordan, the land becomes a barren wasteland devoid of vegetation, except for the area immediately adjacent to the Jordan.  This area extends from a few miles outside Jerusalem all the way through the Jordan Rift Valley until you hit the major settlements in modern day Jordan.  The valley itself is about 10 miles wide (measurements taken from Google Maps).

Your question about how he managed to gain attention in a remote area is a good one.  One thing to keep in mind is that the late Second Temple Period is a time of religious unrest, when many different groups and figures were competing for their vision of Judaism to be the dominant strain.  Sects like the Pharisees and Sadducees, which focused on the study of Torah and Temple piety respectively, were generally centered in more populous areas (especially Jerusalem).  However, there were more than a few other groups who made their home in the wilderness, including John, the Essenes/Qumran community, and eventually the revolutionaries of Masada.  So John's location is not without precedent.  

Bearing this in mind, along with Jerusalem's relative proximity to the area where John is supposed to have been preaching, we can paint a picture of a murmur among the people of Jerusalem about this man and his disciples in the nearby wilderness, enough of a murmur that the religious leaders came out to see what the commotion was all about.  Three out of the four Gospels specifically mention people coming from Jerusalem (John 1:19, Mark 1:4, Matthew 3:5), which makes sense based on what we have said.  What's perhaps more remarkable is not that the people of Jerusalem heard of John, but that Jesus did.  From all the way in Nazareth, it's over 60 miles to get to that part of the country.  Jesus enters this scene as the outsider, the country boy from the backwoods of the Galilee among a crowd of the educated and the urbanites.