Replacement OS for amiga, sort of but maybe one day.
It runs primarily on x86 hardware but also on m68k, PPC and x86_64.
It can be installed to HD but also booted directly from CD/DVD or installed to a USB key.
Space depends on which distribution (and how much software they come with) that you install.
Nightly Builds CD, upto a couple hundred MB
Icaros 1.3 DVD nearly 2 GB
Icaros 1.3 light 700 MB
(there are 2 more distros not listed.)