Hi Natasha,
There are no limitations to the size of a volume that can be mirrored. The larger the mirror, the longer the mirror synchronization process will take, but that's to be expected. :)
The mirror operation can be performed online without interrupting production. Since the system is going to have to be mirroring the volume and keeping up with changes that occur during the mirror operation, there will be a resource hit on the server but nothing that significantly impacts your production applications that make use of this data.
I hope this helps.
- Robert