The size is given as the diameter of the inner white field of the point.
Reference points are marked with the objects that you want to scan on the 3d scanner. This is the most popular method, and it relies on recognizing the positions of 3d stickers that are stuck on a scanned object. Tags can be of different sizes. It all depends on what object you are scanning. The rule is that the larger the item, the bigger the points have to be applied.
The measurement itself is that the scanner software scans the positions of a given marker and scans the scan once a complete polygon model is completed. It is assumed that one scan must see at least 3 reference points to locate it in space. The size of points for a given object is determined by the size of the measurement field and the resolution of the 3d scanner.