ServerMagic 4 & Mirrored Volumes

Here is the deal: We have two volumes of different sizes each mirrored to differnt volumes. I would like to increase the size of one of the volumes.
Is it posible? Do I have to turn off the mirrors first or it is automaticly done? Will the mirrors increase as the volumes change or not? DATA CAN NOT BE LOST!!! I am running Windows NT 4 Server.
Thank YOU!!!