Mirrored LUNs need to be the same size. Vxvm will automatically create a LUN the same size. Note that the underlying disks can be anything, as long as they present sufficient space to veritas.
The size of the underlying physical disk has nothing to do with the veritas volumes on top of it. This is the whole point of virtualising your storage.
Through the GUI or vxadm,
- Add enough diskspace to the diskgroup to facilitate the creation of a mirror. (In your example, find out which diskgroup the 100GB physical disk is in, and add the ten 10GB disks to this group)
- "Mirror volume", and select these newly added disks as the target.
Vxvm will handle the rest.