You are confusing LUNs and Volumes a little in your question.
Volumes are Snapmirrored to volumes. A LUN is nothing more than a file within a volume. So changing the size of a LUN within a volume doesn't require anything to be done on the Snapmirror destination. Snapmirror doesn't care what is in the volume that is being mirrored - it just makes a replica copy. So if you change a LUN (file) on the source the change will replicate to the snapmirror destination on the next update.
Now the caveat of course is if you are resizing the volume that contains the LUN in question, perhaps to reserve space for the newly expanded LUN. In that case, you *may* need to take action. In 7-mode, changing the size of the source volume in a snapmirror requires you to also change the size at the destination. In Clustered Data Ontap, the destination can automatically pick up the new source volume size and adjust accordingly assuming space exists to expand at the destination.
I make the point on terminology only for clarity. LUN mapping refers to making a LUN available to hosts through an igroup. Volumes contain LUNs. Volumes are snapmirrored to a destination or series of destination volumes.