A SizeReporter's only purpose (at the moment) is to calculate the size of a resource. How the size is calculated or what unit it is in is defined by the class implementing this interface. One might use the amount of bytes and another might use the amount of triples stored in a resource.
Calculate the size of a chunk based on which SizeReporter is being used
the size of the passed chunk as a number
the chunk of which you want the size
Estimate the size of a body / request by looking at its metadata
the estimated size of the body / request or undefined if no meaningful estimation can be made
the metadata of the resource you want an estimated size of
Get the unit as a string in which a SizeReporter returns data