Disks did not require to have the same size, you can have one disk of 1Go and one of 500Mo, so according to the mode raid as you chosen, you can have two different partitions of 500 and 1000 Mb, or a single partition of 1500 Mo grouping together both disks as if it ny had that of it the only one.
The only constraint is that both disks are the same type PATA or SATA or SCSI
In the case of a mirroring raid, the maximal size of the partition which can be created is the one of the smallest disk in that case particular. And I shall not speak here about the mode about RAID where the data are fairly distributed on each of the disks because for a good configuration Raid are needed at least 4 disks also sizes if possible.