I'm pretty sure you can't on a Mac.
You CAN install (and boot from) OS X from an external, but OS X is not Windows, so you cannot compare the 2.
And BootCamp DOES say that Windows needs to be installed on your startup disk, but I'm almost positive that refers to the internal startup disk ONLY. Actually.. the documents say that it has to be the internal startup disk.
And finally, Macs don't have BIOS, they have EFI. What I meant earlier is that I don't think booting Windows from an external drive is supported on a Mac.. yet.
EDIT: apparently some people have been able to boot Windows off an external USB drive with minor issues.
https://forums.macrumors.com/threads/191777/
I'd still go for the largest internal.