Question about Garmin Speed sensor 2

The sensor is mounted on the axle of a wheel and counts the number of revolutions. It then calculates the distance travelled in a certain period of time based on the set value of the circumference of the wheel.
I see 2 problems with the spinning bike: 1. Is there an exposed axle to attach it to? 2. what circumference do you set?
For 2. you could of course use the typical circumference of a road bike...