Problem Interpreting METADATA_SIZE header

[hallaf]

I do not understand how to interpret the METADATA_SIZE field.
In the attached example I am sending four fiducial markers from Slicer to a Kuka LBR.
The Metadata size should be 53 but I don't know how to get that out of what is there. 0X 00 12 00 00.
...so I am not able to compute how many markers are coming over.
Have tried other numbers of points with similar results.

Can interpret EXT_HEADER_SIZE just fine...so thought this would be the same.
Thanks!