The "sis undo" command will remove all the sharing done by dedupe and hence each user block will have a dedicated physical block. It will also delete the fingerprint and the changelog files. So there will be no savings after running the undo command. Undo does not restore the layout of previously shared block as they were before, so the layout of the volume will not change significantly after undo.
Taking all this into consideration you cannot alter the size of the volume during the undo operation. You can stop the undo operation, resize your volume, then resume. But you cannot alter sizes while the process is running.