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Shrinking a RAID 0 volume to add a smaller device to it

Suppose we have:

/dev/md3 is the RAID0 device, running in degraded mode.
/dev/sda7 is the surviving component of md0, 921GB in size.
/dev/sdb7 is the new component to be added to md0, 451GB in size.

Get the size of sdb7

Get the exact size (in kb) of the /dev/sdb7 partition. Using parted and display sizes in sector units:

parted /dev/sdb
(parted) unit s
(parted) print
...
Sector size (logical/physical): 512B/4096B
...
 7   95703040s  976773119s  881070080s    home    raid

so the size of /dev/sdb7 is 881070080 / 2 = 440535040 kb.

Get the size of sda7

Get the size (in kb) of the /dev/sda7 partition:

parted /dev/sda
(parted) unit s
(parted) print
...
Sector size (logical/physical): 512B/4096B
...
 7   95703040s  1894920191s  1799217152s    home    raid

The size is 1799217152 / 2 = 899608576 kb.

Get the size of the /dev/md0 RAID volume:

mdadm -D /dev/md3
...
    Array Size : 899476480 (857.81 GiB 921.06 GB)
...

The overhead of the RAID volume is 899608576 - 899476480 = 132096 kb. For safety we add an extra 128 kb for the RAID superblock.