No one geofence size is optimal for all situations. How far away from home do you work, play and shop from your home? Do you want your thermostat to setback when you are at work all day, but not when you run errands on the weekend?
Your geofence boundary is a starting line. When you cross the starting line and are moving away from your home, you start saving energy by setting back your thermostat automatically. Do you want this to happen when you get on the bus or when you reach the parking lot at work?
When you cross the starting line and are moving toward your home, your system starts working to make your home comfortable. Do you want this to happen when you exit the Highway or leave the grocery store parking lot?
To choose your geofence size, consider your priorities - comfort or savings.
Larger geofences will give your system more time to reach your comfort temperature when returning home. Smaller geofences will maximize potential energy savings, but may not give your system the time required to return to a comfortable temperature when returning home.
Consider - Where do you usually go when you leave your home each day? How long do you generally spend there, and how long does it take to get there and back? Some trips will be so short and so close to your home, you may want to keep your thermostat in Home mode. Others will have you away from your home long enough or at a far enough distance that you'll want to save money by going into Away mode.
When you set your geofence size you will see a map of your neighborhood so you can visualize the size and placement of your geofence boundary. You can always come back to your settings and adjust your size any time you need to.