What's the default beacon region size in the unit of meter?

When I use the method startRangingBeaconsInRegion, I am not clear about the region size, how far away from the beacon to our app mobile? Can we readjust the region size?

Region size is not defined in meters. It is defined by iBeacon identifiers (proximity UUID, major, minor). Think more that region is any region that you see within Bluetooth radio range beacons.

See: http://estimote.github.io/iOS-SDK/Classes/ESTBeaconRegion.html