The minimum size of the Virtual Fence is 50 meters (radius) for a circular fence and 100 x 100 meters for a rectangular fence. Virtual Fences cannot be made smaller than this at this time due to the constraints of current GPS technology.
Why can't I make the Virtual Fence smaller?
GPS positions are calculated using several different coordinates estimated by the distance of the tracker to multiple GPS satellites. If the Virtual Fence were to be made smaller, some of these coordinates might fall inside and outside of the safe area, rendering the geofence ineffective.
All GPS devices experience difficulty retrieving coordinates while shielded or covered (for example by the roof of your car or home), due to GPS interference, so the above is especially the case with indoor usage. Thus, the minimum size of the Virtual Fence also helps to prevent false Virtual Fence outbreaks under these circumstances.