You need to have a calibration reference in the video. Add a line drawing on top of something of a known length, right click the line and go into Calibrate measure to set the physical length of the object.
The accuracy of the measure will depend on several factors.
The line of known length should be on the plane of motion otherwise there will be perspective error.
The calibration object should be as large as possible and as close to the center of the image as possible to limit error due to camera lens distortion.
It should also be as close as possible to where the trajectory will take place. One idea would be to use the leg of the kicker if static points are clearly visible on it during the kick and assuming it doesn't deform (see also reservations about rotation).
To limit this camera lens distortion avoid using fisheye type lens by all means. With regular camera you will want to move back and then zoom in, this will flatten the scene and reduce perspective errors. Use a tripod and remote control the recording to minimize camera movement between calibration and measure. This is especially important if you take several attempts with the same calibration frame.
The camera optical axis should be perpendicular to the plane of motion to avoid perspective errors. At the least you will want to make sure it's parallel to the ground, with a small spirit level for example.
I'm not too familiar with how much rotation component is there during the kick. Not all of the motion will take place on a single plane of motion. You need to keep that in mind when comparing speed of several subjects if they have different kicking style.
The next version is coming along and will have several improvements on the topic. (sub pixel tracking, changing tracker parameters, acceleration, angular speed, etc.)
