Does anyone know how much disk space MSDTC requires when adding to a cluster group? Does it depend on the size of the enlisted transactions??

Well, first off, it should be in its own group (with its
own IP, name, and disk). It totally depends on your use
of DTC in terms of size of the disk used ... there is no
one right answer. I'd say may it 500 MB or 1 GB and size
up if needed after some testing.
the enlisted transactions??