using fdisk: show size in a unit such as MB or GB

You can't. Use something else like parted -l instead.

See man parted for more information.

I'm not sure about other OS; but in Debian Jessie it displays in MB & GB by default.

E.g.:

# fdisk -l /dev/sda

Disk /dev/sda: 1 TiB, 1120239009792 bytes, 2187966816 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 1D01BF33-C584-4C49-A05F-341CFB8E2D24

Device         Start        End    Sectors   Size Type
/dev/sda1         34       2047       2014  1007K BIOS boot
/dev/sda2       2048     262143     260096   127M EFI System
/dev/sda3     262144  234441614  234179471 111.7G Linux LVM
/dev/sda4  234442752 2187966782 1953524031 931.5G Linux filesystem

Note the 5th column "Size"; K = KiB; M = MiB & G = GiB