You usually resize the file system at the same times as you resize your virtual device. If you are growing the file system and virtual device, you should grow the device first (with the RAID or LVM tools), and then grow the file system. If you are shrinking the file system and virtual device, you should shrink the file system first, and then the virtual device afterwards.
To resize the file system in Parted, use the resize command. For example:
(parted) select /dev/md0 (parted) resize 1 0 20
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