Geo fence how many feet minimum

When I set-up my geo fence initially , it only allowed a minimum of 500 feet from the location I was in, that 500 feet covered even neighboring houses and roads.

How can I reduce the ge fence feet to 100 feet? Is this a common problem or I am the only one with this problem?

I do believe that is the Min setting for SmartThings. One reason is that GPS is only so accurate, so they have to create a minimum setting in order for them to accurately detect when you leave the area and come back. I normally use the geo fence along with the ST Presence sensor to indicate when I leave and come back.