[WesolyKubeczek]

Says 512 bytes of machine code, not "fits in the MBR". You can only cram 355 or so bytes of code into MBR area designated for that.

[FreeFull]

If you're fine with overwriting the partition table, you can go up to 510 bytes of code, followed by 2 bytes for the mandatory MBR boot signature

[CodeWriter23]

No partition table on floppy disks.