MBR size is 440 bytes or 512 bytes
The MBR (Master Boot Record) is 512 bytes.
- 446 bytes Bootloader
- 64 bytes (4 * 16 bytes) Partition Tables
- 2 bytes Magic Number which is AA55H
However these values are for generic MBR, you can see other MBR structures from: https://en.wikipedia.org/wiki/Master_boot_record
Anyway you have to backup hole 512 bytes of MBR with dd (disk-to-disk) command.
The MBR IS 512 bytes. So the first example is how you would back it up. The partition table is at the end, in the area after 440 bytes in - so, if you wanted to back it up WITHOUT the partition table, then you could use the second example (why you'd want that, I don't know).