Yes you can set the geo-fence to be anything from about 500 ft to 1,636,627 ft (seriously, I just checked) 
Most people complain that the low end cannot be changes to less than 500 ft but we have found with the way phones work these days, that is about a minimum to reduce false changes in presence based on âtower driftâ etc.
Perrrfect- Iâll start with 26,400ft and see how it does! 
Already tried that. From nearly 10000 miles away. 
How do I adjust the radius of my iphoneâs geofence? I remember adjusting the radius of the geofence when I first set up my ST, but Iâve been poking around and I canât figure out how to reduce the radius of my geofence.
From the Dashboard, click the three dots⊠Select My Location from the menu, click on the gear on your location picture and there you are.
I have 5 iPhones setup as presence sensors and a descent sized geofence but, ST never seems to want to recognize when anyone leaves or arrives. Some times it does but most of the time it doesnât so my wife is always setting off the alarms when she comes in Or, the System will finally register some was as present 5 to 10 minutes after they arrive. What can I do to get my presence sensors to work correctly?