pmartyn, zFS is short for z/OS Distributed File Service (DFS) zSeries File System (zFS), which is the replacement file system for HFS in Unix System Services on z/OS systems. See http://www.redbooks.ibm.com/redbooks/pdfs/sg246580.pdf
jaggz, your best bet would be to reference whatever material mentioned maxvol and use it. I've defined and grown plenty of zFS file systems over the years (some up over 40 GB total space) using IOEAGFMT, and the parameters are typically in MB not volumes. As long as the storage group has available volumes with available free space, zFS will convert the space request to cylinders. I've never heard of "maxvol" as a parameter, which is why I recommend you go back to wherever you got that information from.