To ensure you get the best motion capture data in Flipside, it's best to calibrate your tracking so that Flipside knows your height and other dimensions.
This is done by pressing the Calibrate button found on the underside of the Characters palette. Flipside will then instruct you to stand in a T-pose with your arms outstretched at your sides for a few seconds, as shown below.
Flipside will remember your calibration settings for future use, so if you're creating shows with multiple actors in the same headset it's a good idea to calibrate once each time a new actor wears the headset. If you're just using Flipside solo, then you should only need to calibrate once.
Next: Full-body tracking