It is apparent that once a datum plane or axis is mirrored, you can no longer adjust the copied plane/axis size nor does it follow the original if the original is edited. This is true if you don't select the optional Varied mirror.
Either the copy must follow the original after the mirroring if the original is resized or the RMB click should allow resizing specifically.
The lack of allowing editing the copied datums features is a serious oversight.
Preferred functionality is to allow resizing of ALL datum planes and axes from a right mouse click popup menu.
At the very least, the mirrored version should always follow the original by default.
It is -not- an option to re-create the mirror or pattern since this might be far up the tree and would affect a lot of work.