In 1.2.x and 1.3.x, distance to the furthest door is used as a village's radius.
From 1.4.2, its definition seems to be changed. I create a larger (more than 32m
radius) village, NPC villager wanders only central area, not whole village.

I'll attach simplified sample world. In this world, size of a village is 40m radius.
In 1.4.4, when I use spawn egg at 'yellow' area (24m far from center),
a new villager suddenly runs away. At 'blue' area, he/she stays there.

This behavior is same as at a village's edge. In 1.3.2, at 'red' area (41m),
a villager runs, and at a point of 40m, he/she doesn't run.