HI Romick,
The size of the IO blocks are not compared. Mirror Mode driver mirrors the I/O. In other words, when a virtual machine that is being Storage vMotioned writes to disk, the write will be committed to both the source and the destination disk. The write will only be acknowledged to the virtual machine when both the source and the destination have acknowledged the write.
Find more details on the process in following article by Duncan Epping.
http://www.yellow-bricks.com/2011/07/14/vsphere-5-0-storage-vmotion-and-the-mirror-driver/