The zone area should be the total of 7 planters, if you use the same Pixie to control all of them. If your planter is square, the area should be (41/12)*(12/12) = 3.4 square feet, the total is 24 square feet. The zone area is used for estimating how many gallons of water you have used. It will not affect the watering schedules. So an estimated number is good enough usually.
Your whisperer report sensor data to our cloud server, which will adjust your watering schedules automatically.