Depending on the ESXi version, and the vSphere Client you may be able to see the size from the Datastore Browser.
Other than this, enable SSH on the host, and use putty to connect to it. Then go to the folder that contains the orphaned .vmdk file(s), and run ls -lisa.
Please allow me a quick question. How did you find out that the file(s) is orphaned? There are only rare cases when this happens, so ensure that it's really not in use anymore, before you consider to delete it!
André