RAID sets can't be resized wth Disk Utility. Here's what works best, if you have a large number of files to move to the new array (e.g., a Time Machine archive, which can have millions of tiny, individual files.)
Note that you'll need a third drive to boot from, if your RAID array is also your boot drive. A MacOS install DVD or USB stick will do, since they enable running Disk Utility. You will also need at least one external dock or enclosure, for at least one of your old RAID drives.
Format the new drives as Journaled, case-sensitive, not encrypted.
Partition the drives, if desired.
Call up the RAID Assistant within Disk Utililty. Create an new mirrored set with the new drives, or with size-matched partitions on the drives. You can leave non-RAIDed partitions on each drive, and/or create multiple RAID sets from multiple matching partitions. (This is very useful if you have virtual machines, or large movie files that are static and don't generate new versions over time.)
NB: Because a VM is "new" to Time Machine every time you use it,
it will give rise to multiple 30-50GB archived versions that
will very quickly eat into the available Time Machine storage
space. It's best to exclude VMs from backup in Time Machine's
preferences, and back them up manually in their own separate
Another use for a separate partition is to hold a bootable clone
of your startup HD (Carbon Copy Cloner is good for this.) If the
startup drive fails, the clone lets you reboot and carry on within
minutes (and keep operating until your replacement drive arrives.)
Mount at least one of your old RAID drives in an external dock or enclosure.
Here's the key step: In the left pane of Disk Utility, select from "RAID Sets" your desired target RAID, then click the "Restore" button. Select the source volume on the old RAID drive that you want copied to the new RAID set, and let the restore proceed. It may take many hours, but that's far better than the many days that Finder would require.