JDWP specification at https://docs.oracle.com/javase/8/docs/technotes/guides/jpda/jdwp-spec.html says:
Name Size
referenceTypeID same as objectID

While the text at the bottom of the page is calling out referenceTypeID as being sized independently (along with objectID, fieidID, etc):
"Object ids, reference type ids, field ids, method ids, and frame ids may be sized differently in different target VM implementations"
IDSizes JDWP command also specifies referenceTypeID size.

Looks like for the referenceTypeID "Size" column should say
"Target VM-specific, up to 8 bytes (see below)".