Want to extend an XtremIO volume? Simply right click on it and say modify volume. Pretty darn easy. Was somewhat annoyed to find that this information isn’t exactly readily available from “the Google”, so I decided to write this up after verifying it is a non-destructive process from my EMC home boys. This was performed on an XtremIO running on the 3.0.0 build 44 code. Not sure if it will work for shrinking a volume, but perhaps someone can comment up with that information.
elgwhoppo's vNotebook 
XIO 4.0.2 has the “apply” button greyed out if I modify a volume with a smaller size. VMFS5 doesn’t support reducing the size of a datastore either.
“volumes size should be no less than the current volume size”
Awesome, thanks Kevin! I mean, I figured that would be the case, but at least it’s good to know it’s not permitted in the GUI.