I'd rephrase it that way:
```
Sets the default size of tags (used to compute their 3D pose) when not explicitly specified in the configuration
 file. To be accurate, the unit must match the unit used for the camera calibration (usually, millimetres). 

Note that is assumes all the tags have the same size. If tags have different size, you may want to list 
them in the configuration file (see read3DConfiguration()).
```