configure RAID 1 with different size disks with software-raid under ubuntu

i have a dual-boot system with ubuntu 12.04 and window 7 (with a disk of size of 500GB in total, 250GB for ubuntu and 250GB for window 7), is there a way to back up both dual-boot OS to an external disk, and then "recover" both of them in the new dual-boot system?

i want to buy another 1 TB disk to configure a RAID 1 (need 2 separate HDs) with the ubuntu OS of this dual-boot system with software-raid as well, is this possible?

due to the size of the current HD for ubuntu OS is 250GB, and it is better that the size of each of the RAID 1 disks is the same, so i will make the 1 TB new disk to 3 partitions, 250GB, 375GB and 375GB, so each of the 2 RAID 1 disks will have the size of 625GB(= 250GB + 375GB). can i configure the RAID 1 for ubuntu OS in this way?

thanks !