Regions atm are a square shape. I don't see how you can really remove that now that a bunch of cities have started building square areas within their regions, it would screw them up quite a bit, but we'll see haha
Your region depends on your city size. At the moment, if you're a town, you recieve a 150x150 Square Region, or in the case of a City a 180x180 Square, etc. Only you and the people that you have assigned to your regions may edit blocks in that region, and you get a small no-pvp area in your Town Hall, which is a seperate region meaning people who can edit the main region don't have to be able to edit the Town Hall one, it's the choice of the Region Owner (Who is usually the town mayor).
Another interesting note is that creeper explosions cannot damage your region's blocks (Although they'll still hurt you.)