First, I think their is some ambiguity as to what exactly constitutes a region.
Regions are a little nebulous in 1.3. During world generation a noise map is sampled to pick "biome region points". Chunks then assign themselves to the closest "biome region point", which determines the biome of the chunk as well as river placement. Multiple nearby "biome region points" can end up with an identical biome, which renders the borders between them invisible.
In 1.5 the generator will get an overhaul, and regions will be permanent fixtures. I'm aiming to have a single region roughly fill a 1920 x 1080. Individual regions will vary, but that's the target. The regions will also be visible to some extent in the UI, probably though the region map.
Selecting a site to settle would highlight the region claimed, making explicit any borders not already obvious from a coastline or biome change.
That's precisely my plan, it will make things nice and obvious.