The device argument will usually be a disk partition (something like /dev/sdb7) but can also be a file. The Linux kernel does not look at partition IDs, but many installation scripts will assume that partitions of hex type 82 (LINUX_SWAP) are meant to be swap partitions. (Warning: Solaris also uses this type. Be careful not to kill your Solaris partitions.)
The size parameter is superfluous but retained for backwards compatibility. (It specifies the desired size of the swap area in 1024-byte blocks. mkswap will use the entire partition or file if it is omitted. Specifying it is unwise – a typo may destroy your disk.)
After creating the swap area, you need the swapon command to start using it. Usually swap areas are listed in /etc/fstab so that they can be taken into use at boot time by a swapon -a command in some boot script.
mkswap, like many others mkfs-like utils, erases the first partition block to make any previous filesystem invisible.
However, mkswap refuses to erase the first block on a device with a disk label (SUN, BSD, ...).
Also, without this option, mkswap will refuse to erase the first block on a device with a partition table.
The maximum number of the pages that is possible to address by swap area header is 4294967295 (32-bit unsigned int). The remaining space on the swap device is ignored.
Presently, Linux allows 32 swap areas. The areas in use can be seen in the file /proc/swaps
mkswap refuses areas smaller than 10 pages.
If you don't know the page size that your machine uses, you may be able to look it up with "cat /proc/cpuinfo" (or you may not – the contents of this file depend on architecture and kernel version).
To set up a swap file, it is necessary to create that file before initializing it with mkswap, e.g. using a command like
# dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
to create 8GiB swapfile.
Please read notes from swapon(8) about the swap file use restrictions (holes, preallocation and copy-on-write issues).