Master Boot Record (MBR) is a really old Microsoft standard for defining partitions. An MBR label can only occupy the first 512 Bytes of a disk block. Only enough space is left to define four partitions after the boot code. It is possible to have extended partitions by defining more MBRs in one of these 4 partitions, but it is as fucky as it sounds, and cause subtle problems.
MBR used 32 bit addressing to store the addresses of sectors. Each sector can store 512 bytes. So a total size of
MBR allows for one partition to be marked as active, which is where the bootloader looks to load the OS. The DOS command that was used, was also called fdisk, thought on modern Windows, we use diskpart for managing disks.